Greenhouse Gas Fluxes from Soils of Different Land-Use Types in a Hilly

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Agriculture, Ecosystems and Environment 124 (2008) 125–135 www.elsevier.com/locate/agee

Greenhouse gas ﬂuxes from soils of different land-use types in a hilly area of South China

Hui Liu a,b, Ping Zhao a,*, Ping Lu c, Yue-Si Wang d, Yong-Biao Lin a, Xing-Quan Rao a a South China Botanic Garden, Chinese Academy of Sciences, Guangzhou 510650, PR China b School of Tourism and Environment, Guangdong University of Business Studies, Guangzhou 510320, PR China c EWL Sciences, P.O. Box 39443, Winnellie, Northern Territory 0821, Australia d Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, PR China

Received 15 October 2006; received in revised form 3 September 2007; accepted 11 September 2007 Available online 24 October 2007

Abstract

The magnitude, temporal, and spatial patterns of greenhouse gas (hereafter referred to as GHG) fluxes from soils of plantation in the subtropical area of China are still highly uncertain. To contribute towards an improvement of actual estimates, soil CO2,CH4, and N2O fluxes were measured in two different land-use types in a hilly area of South China. This study showed 2 years continuous measurements (twice a week) of GHG fluxes from soils of a pine plantation and a longan orchard system. Impacts of environmental drivers (soil temperature and soil moisture), litter exclusion and land-use (vegetation versus orchard) were presented. Our results suggested that the plantation and orchard soils were weak sinks of atmospheric CH4 and significant sources of atmospheric CO2 and N2O. Annual mean GHG fluxes from soils of plantation 1 1 1 1 and orchard were: CO2 fluxes of 4.70 and 14.72 Mg CO2–C ha year ,CH4 fluxes of 2.57 and 2.61 kg CH4–C ha year ,N2O fluxes 1 1 of 3.03 and 8.64 kg N2O–N ha year , respectively. Land use types had great impact on CO2 and N2O emissions. Annual average CO2 and N2O emissions were higher in the orchard than in the plantation, while there were no clear differences in CH4 emissions between two sites. Our results suggest that afforestation could be a potential mitigation strategy to reduce GHG emissions from agricultural soils if the observed results were representative at the regional scale. CO2 and N2O emissions were mainly affected by soil temperature and soil moisture. CH4 uptakes showed significant correlation with soil moisture. The seasonal changes in soil CO2 and N2O fluxes followed the seasonal weather pattern, with high CO2 and N2O emission rates in the rainy period and low rates in the dry period. In contrast, seasonal patterns of CH4 fluxes were not clear. Removal of surface litter reduced soil CO2 effluxes by 17–25% and N2O effluxes by 34–31% in the plantation and orchard in the second sampling year but not in the first sampling year which suggested micro-environmental heterogeneity in soils. Removal of surface litter had no significant effect on CH4 absorption rates in both years. This suggests that microbial CH4 uptake was mainly related to the mineral soil rather than in the surface litter layer. # 2007 Elsevier B.V. All rights reserved.

Keywords: GHG ﬂux; Orchard; Pine plantation; Litter exclusion; Soil moisture; Soil temperature

1. Introduction global warming are also made by methane, CH4,and nitrous oxide, N2O. Soils can store and release consider- Gas exchange between soils and the atmosphere is an able quantities of carbon through natural processes important contributing factor to global change due to including litter deposition, decomposition and root increasing release of greenhouse gas (GHG) (Bouwman, respiration (Drewitt et al., 2002). Whereas, forest soils 1990). The most important individual greenhouse gas is have been identified as a significant sink for atmospheric carbon dioxide, CO2, but substantial contributions to CH4, and it is estimated that CH4 uptake activities of soils represent 3–9% of the global atmospheric CH4 sinks * Corresponding author. Tel.: +86 20 37252881; fax: +86 20 37252831. (Prather et al., 1995). Soils have also been identified to be E-mail address: [email protected] (P. Zhao). significant sources for N trace gases, accounting for 60% of

0167-8809/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2007.09.002 126 H. Liu et al. / Agriculture, Ecosystems and Environment 124 (2008) 125–135 the total annual N2O emissions (Ehhalt et al., 2001; 2. Materials and methods Maljanen et al., 2001). Researchers have emphasized the importance of improving our understanding of soil 2.1. Experimental sites and materials processes in order to gain more confidence in projections about future changes in the global atmospheric GHG The study was carried out at the Heshan Hilly Land concentrations (Prather et al., 1995; Drewitt et al., 2002; Interdisciplinary Experimental Station of the Chinese Merino et al., 2004). Academy of Sciences in the subtropical region of China Land use practices have great impact on GHG flux from (Heshan, Guangdong Province, China, 1128540E, 228410N). soil surface (Dobbie et al., 1996; Prieme´ and Christensen, The mean annual temperature is 21.7 8C, the mean rainfall is 1997; Smith et al., 2000; Houghton, 2002). In China and 1700 mm, and the mean evapotranspiration is 1600 mm many other countries, degraded land is increasingly (http://www.scib.ac.cn/hsz/English/index.htm). converted into pasture or newly planted forest, which A pine plantation (20-year-old Pinus massoniana trees) noticeably increases the total grassland and forest areas. In and a longan orchard (12-year-old Dimocarpus longan Lour China, afforestation area was 45 106 ha in the past 30 trees) were selected to evaluate the effects of land use on the years (Houghton, 2002), which includes the planting of trees soil GHG exchanges. Prior to the plantings, both the pine for timber as well as shelterbelts, fuel wood, and orchards. plantation and orchard sites had similar soil characteristics Such changes can substantially alter soil organic carbon (Li et al., 2000). There was a layer of 3–5 cm thick of half- dynamics (Li et al., 2002) and affect exchanges of GHG decomposed litter and coarse woody debris under the pine between the soil and the atmosphere (Zhou et al., 2004). plantation (Shen et al., 2001). The selected sites were However, effects of land use on changes of GHG emissions representative of the regional features of land use in hilly from soils in subtropical area of South China are poorly area of South China. The soil is an oxisol developed from understood. Since the subtropical climate is characterized sandstone and the main characteristics of the soil at the two typically by rainy and dry seasons, seasonal patterns of the sites are listed in Table 1. There were no differences in GHG emissions are important for our understanding of how texture and C/N ratio however higher pH value, soil organic soil temperature and soil moisture affect soil C and N C, soil microbe C and total nitrogen but lower bulk density in turnover processes and associated trace gas emissions in the the orchard soil than in the pine plantation soil. subtropical region. The objective of the present work was to: (1) investigate 2.2. Experimental design and treatments seasonal changes in the GHG fluxes from soils of a pine plantation and an orchard; (2) quantify annual total soil In March 2003, six GHG gas exchange chambers were GHG fluxes; (3) evaluate the responses of GHG fluxes to installed ineach of the pine plantation andthe orchard. At each litter exclusion and key driving variables, such as soil site, three chambers were randomly designated to measure the temperature and soil moisture. impacts of surface litter exclusion (i.e. the bare soil or ‘BS’

Table 1 Soil characteristics at the two experimental sites (pine plantation and orchard) in Heshan Land use Pine plantation Orchard Soil texturea Sandy clay loam Sandy clay loam Litter accumulation (t h m2)b 18.7 – Litter input (t h m2 year1)b 7.30 – Soil organic C (gC or g kg1 dry soil)c 11.89 13.40 1 c Soil microbe C (mg Cmic (100 g) dry soil 29.68 47.79 Soil organic carbon storage (t h m2)d – 74.66 Bulk density (g cm3)e,a 1.11 0.99 pHa,f 3.95 5.91 Total nitrogen (g kg1)e,a 1.19 1.49 1 g NO3 –N content (mg kg ) 4.23 – + 1 g NH4 –N content (mg kg ) 6.28 – C:Nf,a 10 10 Total phosphorus (g kg1)a – 0.98 Available phosphorus (mg kg1)a – 35.8 3 3 a Maximum water holding capacity (cm H2Ocm soil 100) – 38.6 a Li et al. (1995). b Shen et al. (2001). c Zhou et al. (2004). d Li et al. (2002). e Value from sample collected in March 1999 from 0 to 20 cm depth of soil. Unpublished data from Heshan Station, 1999. f Wen et al. (2000). g Value from sample collected in September 2002 from 0 to 20 cm depth of soil. Unpublished data from Heshan Station, 2002. H. Liu et al. / Agriculture, Ecosystems and Environment 124 (2008) 125–135 127 treatment), and the rest were used as the control (i.e. soil with (n = 3 in Year 1, n = 4 in year 2) on each sampling day. All surface litter or ‘SL’ treatment). For the BS treatment, litter statistical analyses were performed using SPSS 12.0 was removed carefully at least 1 h before each sampling. Field software package (SPSS Inc., Chicago, USA). The normal measurements were carried out twice a week from 21 March distribution of the data was tested by the Kolmogorov– 2003 to 20 April 2005. The stainless steel bases were moved Smirnov test. For normally distributed data, analyses of after 11 March 2004 in the pine plantation and after 31 March variance (ANOVA) were performed using daily means to 2004 in the orchard. During the second year of measurement test the difference of soil temperature, CO2 and CH4 fluxes (Year 2), an additional chamber base was installed at each of by season, surface litter treatment (BS and SL), and land use. the two sites (four repetitions in Year 2). A full general linear model (GLM in SPSS) in which land use type was treated as an independent variable was used to 2.3. GHG gas sampling and analysis compare the differences of environmental factors and GHG fluxes between the two types of land use, and to assess the Fluxes of CO2,CH4, and N2O were measured using static significance of the impacts of land use, season, surface litter chamber and gas chromatography techniques (Wang and removal, and their interactions on CO2 and CH4 fluxes. Wang, 2003). The static chamber assembly consisted of a In addition, a reduced GLM model was developed for permanently installed stainless steel base (50 cm 50 cm each land use to assess the significance of the effects of 10 cm) with a U-shaped groove at the top edge to hold a season, surface litter treatment, and their interactions on mobile stainless steel cover (50 cm 50 cm 50 cm). Once CO2 and CH4 fluxes. The relationships between CO2 fluxes the cover was placed onto the base, the groove was filled with and soil temperature were examined using model described water to a depth of 2 cm, which acted as an air seal. Two in equation (1). A p-value <0.05 was used to reject the null battery-operated fans inside the stainless steel box homo- hypothesis that the model is not significant. As N2Oflux genized the air in the chamber. The cover was also fitted with from bare soil of orchard and soil moisture under the two three temperature sensors and a three-way sampling stopcock. treatments at both sites were non-normal distributed in Year No vent was installed in the chamber. A white thermal- 2, the non-parametric two-sample Kolmogorov–Smirnov insulation cover was added outside of the stainless steel cover test was performed here instead. The Spearman’s r to reduce the impact of direct radiative heating during correlation coefficients between GHG fluxes and soil sampling. A typical measurement started at 09:00 h (a moisture, soil temperature under two treatments of both representative time in this region according to Tang et al., land use types were used here. 2006) and lasted for about 30 min. As for the relationship between soil surface CO2 flux and Gas samples (100 mL each) were collected every 10 min soil temperature and soil moisture, multiple regression using 100 mL plastic syringes. CO2,CH4, and N2O analyses were performed using the stepwise procedure in concentrations in the samples were analyzed in the SPSS (SPSS 12.0 Inc., Chicago, IL, USA) at the p = 0.05 laboratory within 24 h following sampling using gas significance level. chromatography. The gas chromatography configurations and calculation of the fluxes of each gas followed those described by Wang and Wang (2003). GHG flux was 3. Results calculated based on the rate of change in GHG concentration within the chamber, which was estimated as the slope of 3.1. Environmental variables linear regression between concentration and time. All the coefficients of determination (r2) of the linear regression Over the 2-year-study period, the annual rainfall (April– were greater than 0.95 in our study. March) was very similar (1040 and 1035 mm) which was During the gas sampling, soil temperatures at 5 cm below much lower than long-term average annual rainfall surface were measured with a JM624 portable digital (1700 mm). Rainfall had very strong seasonality, with about thermometer (JinMing instrument Co. Ltd., China). Volu- 80% of the rainfall occurring during the rainy season (April– 3 3 metric soil moisture (cm H2Ocm soil) was measured September) (Fig. 1A). There were intense rainstorms in June concurrently at 5 cm depth using a soil moisture meter and August 2003, and in May and August 2004. Annual air (MPKit, ICT Australia). Soil moisture measurement began temperature was 21.4 8C, with monthly temperature ranging on 1 July 2003. Climatic data (precipitation and air from 28.4 8C (July 2003) to 13.1 8C (January 2004) in year 1 temperature) were obtained from the weather station at and annual air temperature 21.6 8C, with monthly tempera- Heshan Hilly Land Interdisciplinary Experimental Station, ture ranging from 27.8 8C (August 2004) to 13.2 8C (January part of the Chinese Ecosystem Research Network. 2005) in year 2 (Fig. 1A). Soil temperature showed clear seasonal variation with 2.4. Data analysis monthly temperature ranging from 7.7 8C (5 February 2004) to 30.1 8C (1 July 2004) in the pine plantation (Fig. 1B) and GHG fluxes, soil temperature, and soil moisture for each from 8.9 8C (9 February 2004) to 30.9 8C (8 July 2003) in treatment were calculated by averaging the 3–4 replicates the orchard (Fig. 1C). Soil was warm in rainy season and 128 H. Liu et al. / Agriculture, Ecosystems and Environment 124 (2008) 125–135

Fig. 2. Seasonal patterns of CO2 ﬂuxes measured at the sites of (A) pine, (B) orchard with (SL) or without (BS) surface litter from March 2003 to March 2005. Each datum is mean of three replications in year 1 and four replications in year 2.

treatment in year 1 and in both treatments in year 2 ( p < 0.05) (Table 2).

3.2. CO2 ﬂuxes

At both sites, CO2 emissions were significantly higher in rainy season (mean value from 194.40 to 2 1 869.91 mg CO2 m h ) than that in dry season (mean 2 1 value from 112.81 to 657.68 mg CO2 m h )(p < 0.001) (Fig. 2A and B; Tables 2–4). Seasonality of CO2 emissions was more pronounced in the control (SL) treatment than in the litter exclusion (BS) treatment at both sites (Fig. 2A and B). Multiple regression models for the relationship between Fig. 1. Seasonal patterns of (A) air temperature and precipitation at Heshan station, (B and C) soil temperature, and (D and E) volumetric soil moisture soil CO2 emissions, soil temperature and soil moisture in measured in the pine plantation and orchard, respectively, with (SL) or year 2 were shown in Table 5. The interaction of soil without (BS) surface litter from March 2003 to March 2005. Each datum temperature and soil moisture could explain 44–75% of CO2 (B–E) is mean of three replications in year 1 and four replications in year 2. emission variations from the control (SL) treatment (Table 5). CO2 emissions from the litter exclusion (BS) treatment were mainly affected by soil temperature. cool in dry season (Fig. 1B and C; Table 2). Soil moisture CO2 fluxes were in the range from 56.57 to 3 3 2 1 ranged from 4.67 cm H2Ocm soil (11 November 2005) 370.38 mg CO2 m h in year 1 and from 27.38 to 3 3 2 1 to 36.91 cm H2Ocm soil (6 August 2003) in the pine 426.57 mg CO2 m h in year 2 in the pine plantation; 3 3 2 1 plantation and from 4.70 cm H2Ocm soil (29 December from 105.12 to 988.43 mg CO2 m h in year 1 and from 3 3 2 1 2004) to 41.51 cm H2Ocm soil in the orchard (26 August 54.99 to 1849.33 mg CO2 m h in year 2 in the orchard 2003), which was consistent with rainfall and was higher in (Fig. 2A and B). Annual average fluxes (mean S.E.) for 2 rainy season (April–September) than that in dry season years from plots with litter were 4.70 0.19 Mg CO2– 1 1 (October–March) (Fig. 1D and E; Table 2). Cha year in the plantation and 14.72 0.73 Mg CO2– Litter removal had no significant impact on soil Cha1 year1 in the orchard. Land use had a significant temperature and soil moisture at any given site (Tables 2– impact on CO2 efflux. CO2 effluxes from pine plantation 4). Soil moisture in the pine plantation was significantly were significantly higher than those from orchard drier than that in the orchard in the litter exclusion (BS) ( p < 0.001) (Tables 2–4). H. Liu et al. / Agriculture, Ecosystems and Environment 124 (2008) 125–135 129 0.58 0.55 0.37 1.08 b 1.18** 0.53** 44.50 66.32 56.82 0.014 b 0.010** 0.019** 0.008 0.007 0.006 O fluxes from 2 0.023 0.060 0.040 0.58 20.24 0.55 15.52 0.38 24.55 1.05 b 19.29 1.13** 11.08 0.50** 26.93 38.07 772.33 36.59 657.68 44.24 869.91 0.010 b 0.220 0.005** 0.132 0.015** 0.302 0.007 0.008 0.006 0.015 0.050 0.031 0.63 20.19 0.61 15.47 0.56 24.50 1.30 a 21.10 1.49** 12.96 0.91** 28.66 24.13 575.55 34.86 328.30 29.73 785.98 0.007 a 0.152 0.009** 0.094 0.009** 0.205 0.006 0.006 0.005 S.E.). 0.028 0.051 0.039 tment in the same year denote significantly different rates (two- 0.64 20.70 0.65 16.70 0.56 24.71 1.14 b 23.02 1.33** 17.61 0.93** 30.66 19.72 419.60 24.88 350.80 29.95 486.42 0.008 b 0.059 0.007** 0.041 0.012** 0.080 0.008 0.006 0.005 0.026 0.036 0.031 0.58 20.66 0.57 16.67 0.43 24.66 0.99 a 24.67 0.78** 20.32 0.57** 31.10 11.58 402.93 7.52 372.38 11.60 432.61 0.005 a 0.072 0.005** 0.038 0.007** 0.109 0.007 0.005 0.005 0.020 0.070 0.043 0.57 21.73 0.56 16.92 0.43 25.97 0.91 a 17.55 0.68** 9.46 0.46** 25.25 8.74 205.36 7.95 118.18 9.60 290.56 0.007 0.003 a 0.064 0.004 0.038 0.003 0.089 0.006 0.005 0.022 0.059 0.039 0.001. **Along the columns within the same year denote significance of the impacts of season on soil moisture and N 0.75 21.50 0.79 16.84 0.62 25.60 1.50 a 15.43 1.49** 8.09 0.87** 22.43 9.90 170.45 11.05 112.81 14.00 226.79 0.009 0.005 a 0.042 0.007** 0.035 0.006** 0.049 0.005 0.005 < a 0.032 0.035 0.032 0.76 21.24 0.79 17.35 0.62 25.71 1.48 a 22.94 1.33** 18.12 0.94** 32.21 9.46 182.99 13.07 150.44 13.68 220.56 0.008 0.007 a 0.038 0.010** 0.027 0.007** 0.053 0.005 0.004 0.033 0.035 0.034 Pine plantationYear 1BS SL Year 2 BS SL Orchard Year 1 BS SL Year 2 BS SL Annual mean 21.05 Dry season 17.03 Annual mean 20.67 Dry season 15.58 Rainy season 30.45 Annual mean 183.87 Dry season 174.74 Annual mean 0.038 Dry season 0.029 Dry season Annual mean ) Rainy season 0.052 ) Rainy season 194.40 ) Rainy season 1 1 1 h h h 2 2 2 m m C) Rainy season 25.66 soil) 8 2 4 3 Om 2 Ocm 2 H 3 flux (mg CO flux (mg CH 2 4 O flux (mg N (cm 2 Soil moisture Soil temperature ( Table 2 Effects of surface litter removal on soil temperature, moisture and GHG fluxes from soil surface of pine plantation and orchard in Heshan (mean CO CH Treatment: BS, bare soil; SL, soil with surface litter. **Significant impact at sample Kolmogorov–Smirnov test). soil surface under different types of land use in Heshan (two-sample Kolmogorov–Smirnov test). Different letters along the rows within the same trea N 130 H. Liu et al. / Agriculture, Ecosystems and Environment 124 (2008) 125–135

Table 3

Significance of the impacts of land use type, litter removal treatment, season, and their interactions on soil temperature, CO2 and CH4 fluxes from soil surface in Heshan (GLM test) 2 1 2 1 Soil temperature (8C) CO2 (mg CO2 m h )CH4 (mg CH4 m h ) Year 1 Year 2 Year 1 Year 2 Year 1 Year 2 Land use ns ns ** ** ns ns Treatment ns ns ns ** ns ns Season ** ** ** ** ns ** Land use treatment ns ns ns ** ns ns Land use season ns ns ns ** ns ns Treatment season ns ns ns ns ns ns Land use treatment season ns ns ns ** ns ns **Significant impact at a < 0.001. ns, no significant impact.

Table 4

Significance of the impacts of litter removal treatment, season, and their interactions on soil temperature, CO2 and CH4 fluxes from soil surface within each of the land use types in Heshan (GLM test) 2 1 2 1 Land use Soil temperature (8C) CO2 (mg CO2 m h )CH4 (mg CH4 m h ) Year 1 Year 2 Year 1 Year 2 Year 1 Year 2 Pine plantation Treatment ns ns ns ** ns ns Season ** ** ** ** ns ** Treatment season ns ns ns ** ns ns Orchard Treatment ns ns ns ** ns ns Season ** ** ** ** * ** Treatment season ns ns ns * ns ns *Significant impact at a < 0.05, and **significant impact at a < 0.001. ns, no significant impact.

Removal of the litter layer reduced soil CO2 emissions in (Tables 2–4). Litter removal and land use did not affect CH4 the second sampling year but not in the first sampling year uptake ( p > 0.05) (Tables 3 and 4). (Fig. 2A and B; Tables 2–4). ANOVA showed that the effect CH4 fluxes were in the range from 0.106 to 2 1 of surface litter removal was significant at both sites in the 0.050 mg CH4 m h in year 1 and from 0.180 to 2 1 second sampling year ( p < 0.001) (Tables 3 and 4). On 0.076 mg CH4 m h in year 2 in the pine plantation; from 2 1 average, the contributions of the litter layer to CO2 efflux in 0.127 to 0.063 mg CH4 m h in year 1 and from 2 1 the second sampling year were 17% and 25% of the CO2 0.156 to 0.144 mg CH4 m h in year 2 in the orchard effluxes in the pine plantation and in the orchard, (Fig. 3A and B). Annual average absorptions (mean S.E.) respectively. for 2 years from plots with litter were 2.57 0.24 kg CH4– 1 1 Cha year in the plantation and 2.61 0.25 kg CH4– 1 1 3.3. CH4 fluxes Cha year in the orchard. At both sites soils on an annual basis acted as a sink for atmospheric CH4. However, we There were no significant seasonal differences in CH4 observed CH4 emissions during heavy rainfall. Land use did uptake in the plantation in year 1 ( p > 0.05), whereas, not affect CH4 uptake ( p > 0.05) (Tables 3 and 4). seasonality had a significant impact on CH4 uptake in the CH4 uptake showed no significant correlation with soil orchard (Tables 3 and 4). In year 2, there were significant temperature at both sites in year 1 ( p > 0.05), while differences at both sites ( p < 0.05) (Fig. 3A and B; Tables 3 significant correlation was observed in both treatments in the and 4). CH4 uptakes were significantly higher in the dry plantation in year 2 ( p < 0.05) (Table 6). In year 1, CH4 season than those in rainy season in year 2 ( p < 0.001) uptake at both sites showed significant negative correlation

Table 5

Multiple regression models for the relationship between soil CO2 emissions (Y), soil temperature at a depth of 5 cm (T) and soil moisture (SM) in year 2 Treatment n Equation Adjusted R2 p Plantation (SL) 77 Y = 12.43T + 3.37SM 123.79 0.75 <0.001 Plantation (BS) 77 Y = 11.86T 85.79 0.67 <0.001 Orchard (SL) 78 Y = 67.07T 22.74SM 162.61 0.44 <0.001 Orchard (BS) 78 Y = 49.41T 439.30 0.61 <0.001 BS, bar soil; SL, soil with litter. Regressions signiﬁcant at p < 0.05 level are shown. H. Liu et al. / Agriculture, Ecosystems and Environment 124 (2008) 125–135 131

Fig. 3. Seasonal patterns of CH4 fluxes measured at the sites of (A) pine, (B) Fig. 4. Seasonal patterns of N2O fluxes measured at the sites of (A) pine, orchard with (SL) or without (BS) surface litter from March 2003 to March (B) orchard with (SL) or without (BS) surface litter from March 2003 to 2005. Each datum is mean of three replications in year 1 and four March 2005. Each datum is mean of three replications in year 1 and four replications in year 2. replications in year 2. with soil moisture ( p < 0.05) in the control (SL) treatment, sampling years except for in the litter exclusion (BS) but not in the litter exclusion (BS) treatment. In year 2, CH4 treatment in the pine plantation in year 2 (Table 2). uptake in both treatments at both sites showed significantly N2O fluxes were in the range from 0.020 to 2 1 negative correlation with soil moisture ( p < 0.01) (Table 6). 0.208 mg N2Om h in year 1 and from 0.046 to 2 1 Litter removal did not affect CH4 uptake ( p > 0.05) 0.256 mg N2Om h in year 2 in the pine plantation; 2 1 (Tables 3 and 4). from 0.026 to 0.294 mg N2Om h in year 1 and from 2 1 0.020 to 0.734 mg N2Om h in year 2 in the orchard 3.4. N2O fluxes (Fig. 4A and B). Annual average fluxes (mean S.E.) for 2 years from plots with litter were 3.03 0.22 kg N2O– 1 1 At both sites, soils were N2O sources (Fig. 4A and B; Nha year in the plantation and 8.64 0.61 kg N2O– 1 1 Table 2). Clear seasonality of N2O fluxes was found in both Nha year in the orchard. Impact of the type of land use treatments at both sites in year 2 (Fig. 4A and B). N2O fluxes on N2O emissions was significant in the litter exclusion (BS) were higher in rainy season than in dry season (Fig. 4A and treatment in two sampling years and in the control (SL) B; Table 2). Two-sample Kolmogorov–Smirnov test showed treatment in year 2 ( p < 0.05), but was not in the control significant seasonal variations ( p < 0.01) at both sites in two (SL) treatment in year 1 (Table 2).

Table 6 Spearman’s r correlation coefficients between GHG fluxes and soil moisture and soil temperature under treatments of two types of land use Year Land use Treatment Spearman’s r 3 3 Soil temperature (8C) Soil moisture (cm H2Ocm soil) 2 1 2 1 2 1 2 1 CH4 (mg CH4 m h )N2O (mg N2Om h )CH4 (mg CH4 m h )N2O (mg N2Om h ) Year 1 Pine plantation BS 0.09 0.28 0.007 0.43* Pine plantation SL 0.19 0.32* 0.46** 0.38* Orchard BS 0.06 0.40** 0.12 0.41** Orchard SL 0.18 0.33* 0.39** 0.47** Year 2 Pine plantation BS 0.32** 0.18 0.46** 0.25* Pine plantation SL 0.40** 0.53** 0.51** 0.60** Orchard BS 0.22 0.60** 0.39** 0.65** Orchard SL 0.24* 0.65** 0.37** 0.65** *Correlation is significant at the 0.05 level, two-tailed; **correlation is significant at the 0.01 level, two-tailed. 132 H. Liu et al. / Agriculture, Ecosystems and Environment 124 (2008) 125–135

ThisstudyshowedthatN2O fluxes were significantly Soil temperature and water content are known to have a correlated with soil temperatures in the control (SL) pronounced influence on the seasonal dynamics of soil treatment in the pine plantation and in both treatments in respiration (Longdoz et al., 2000; Qi and Xu, 2001; Xu and the orchard in two sampling years ( p < 0.05), but not in Qi, 2001). Multiple regression analyses showed that the the litter exclusion (BS) treatment in the plantation seasonal variation in soil CO2 emission was primarily (Table 6). Soil moisture significantly affected N2O controlled by soil temperature and soil moisture in the emission in both treatments at both sites ( p < 0.05) normal state (SL) which could explain 44–75% of the (Table 6). Thus, the seasonal pattern of N2Oemissionswas variations, while soil moisture alone had a weaker effect correlated with precipitation. Pulses of N2Oemissions with the litter removed plots (Table 5). According to were observed after rainfall events. And CO2 emission Dannenmann et al. (2007), the estimated contribution of always increased after rainfall. The Spearman’s r litter respiration to total soil respiration is mainly guided by correlation coefficients showed significant correlations litter moisture. This might provided a reasonable explana- between CO2 emissions and N2O emissions in year 2 (SL: tion for the seasonal CO2 emission variations in our study. r =0.52,p < 0.01; BS: r =0.62,p < 0.01; data not shown Generally, the removal of the litter and organic layers in table) but not in year 1. reduces soil CO2 flux. There seemed to be also a seasonality Two-sample Kolmogorov–Smirnov test showed litter (year 2) of differences between CO2 respiration of BS and removal did not affect N2O emissions in year 1 ( p > 0.05), SL which was also different across sites. Orchard had even but significantly in year 2 ( p < 0.01) (Table 2). On average, higher CO2 emissions at BS versus SL at the end of year 2. In the contributions of the litter layer to N2O efflux in year 2 a wet tropical forest in Puerto Rico, Li et al. (2004) found were 34% and 31% of the N2O effluxes from soil in the pine that litter exclusion had a greater effect on soil CO2 efflux in plantation and in the orchard, respectively. the plantation than in the secondary forest with a reduction of 68% and 54%, respectively. Our results showed that litter removal reduced CO2 effluxes (17% and 25%) from both 4. Discussion sites in year 2. It was lower than the results of Li et al. (2004), but close to pine forest (23%) in the same region (Tang et al., 4.1. Effects of land use, environmental variables and 2006). It is not clear to us why litter removal had no impact litter removal on CO2 fluxes in year 1. It might be due to variation in micro-site characteristics or plant roots considering that gas sampling Annual mean CO2 fluxes (over 2 sampling years) positions were changed after the first sampling year. Some from soils (mean S.E.) were 4.70 0.19 Mg CO2– studies reported higher contributions from root than from Cha1 year1 from the pine plantation, in Heshan, which microbial respiration (Helal and Sauerbeck, 1991; Thierron was similar to the result from a pine forest in the same region and Laudelout, 1996). 1 1 (5.1 Mg CO2–C ha year , Tang et al., 2006). CO2 efflux from the orchard in Heshan was 14.72 0.73 Mg CO2– 4.2. Effects of land use, environmental variables and 1 1 Cha year , which was much higher than those from the litter removal on CH4 fluxes pine forest at Heshan and the natural monsoonal evergreen broad-leaved forest in the same region (9.9 Mg CO2– Annual mean CH4 absorptions over two sampling years 1 1 Cha year , Tang et al., 2006). And the range of CO2 by soils at the pine plantation and orchard (mean S.E.) 1 1 flux from pine plantation was lower than that from orchard were 2.57 0.24 and 2.61 0.25 kg CH4–C ha year , (Fig. 2A and B). These results suggest that conversions of respectively. The annual mean value and the range of CH4 land use would potentially alter the soil-to-atmosphere C absorption were similar at these two sites. It showed land use flux. did not affect CH4 uptake ( p > 0.05) (Tables 3 and 4). And The efflux of CO2 from the soil is in principle the result of the annual mean values are within the range of 0.3– 1 1 two processes: the production and the transport of CO2.In 9.6 kg CH4–C ha year for tropical and subtropical forest soil, plant roots and soil microbes are the dominant forest soils (Smith et al., 2000; Ishizuka et al., 2002), but CO2 producers (Moreń and Lindroth, 2000). Soil organic C lower than that from forests in the nearby Dinghushan was slightly higher in the orchard and microbe C in our study Nature Reserve in the same region (3.4 kg CH4– was higher in the orchard than those in the pine plantation Cha1 year1, Tang et al., 2006). Our results showed that (Table 1). Higher CO2 efflux in the orchard could be partly a the pine plantation and orchard soils were both sinks for result of higher soil microbial biomass. According to Saiz’ atmospheric CH4. study, the organic layer thickness was the only variable that The fluxes of CH4 are influenced by soil variables that yielded significant regressions for explaining spatial influence microbial activity, such as pH and concentrations variation in soil respiration four Sitka spruce stands (Saiz of O2, which, in turn, are controlled by a combination of soil et al., 2006). Future study should evaluate the difference of properties (soil moisture, texture, structure) and soil litter between the pine plantation and orchard and effect of management practices (Merino et al., 2004). Similar bulk roots on total soil CO2 efflux. density in the pine plantation and the orchard (Table 1) H. Liu et al. / Agriculture, Ecosystems and Environment 124 (2008) 125–135 133 suggested that soil gas diffusivity in these soils could be N2O fluxes were positively correlated with soil similar, leading no difference of CH4 oxidation rate between temperature at both study sites and soil moisture in the two sites. orchardinourstudy(Table 6). Soil moisture and soil In our study, CH4 consumption exhibited distinct positive temperature could explain most of the temporal variation correlation with soil moisture (Table 6). This is in contrast to within a site, which was in agreement with the finding from Borken et al.’s (2006) findings that soil moisture strongly a similar study by Pilegaard et al. (2006). In addition, the controlled the uptake of atmospheric CH4 by limiting the seasonal pattern of N2O emissions was correlated with diffusion of CH4 into the soil, resulting in a negative precipitation. Pulses of N2O emissions were observed after correlation between soil moisture and CH4 uptake rates rainfall events, which is consistent with Butterbach-Bahl under most non-drought conditions. Soil moistures in our et al. (2003)’s finding. And the same trend was always study sites were often below the field capacity. CH4 uptake, detected in CO2 emission. It might be due to anaerobic thus, was reduced by limited microbial activity, as a result of microsites formed in soil with frequent rain, being favorable comparable low soil moisture (Dannenmann et al., 2007). to the microbial process of denitrification and soil CH4 is formed in soils by the microbial breakdown of respiration. According to Zhou et al. (2005), in the pine organic compounds in strictly anaerobic conditions (Smith plantation in Heshan, N2O concentrations in different soil et al., 2003). Production of CH4 does not begin until profiles were higher in rainy season than in dry season. reduction of molecular oxygen, nitrate, iron(III), mangane- Higher N2O concentration in soil caused higher soil surface se(IV) and sulphate (all of which maintain a higher emission. potential) is complete (Smith et al., 2003). Such low redox Similar to CO2 efflux, litter removal reduced N2O conditions usually require prolonged waterlogging, as is effluxes (34% from pine plantation floor and 31% from common in natural wetlands and flooded rice fields, as well orchard floor) in year 2, which is in agreement with a similar as in lake sediments (Smith et al., 2003). Soil moisture at our study of Xiao et al. (2004) in a temperate broad-leaved study sites probably did not create such an anaerobic Korean pine forest in North China. conditions needed to affect the activities of CH4 consuming microbes during most of the study period. No distinct changes in CH4 fluxes were found after the 5. Conclusions litter layer was removed (Tables 3 and 4). It suggested that the majority of methane oxidation occurred in the mineral Soil CO2 and N2O emissions in the normal condition soil rather than in the surface litter in the pine plantation and (with litter) at both sites were affected both by soil orchard at Heshan. It is consistent with Borken et al.’s (2006) moisture and soil temperature. Driven by seasonality of findings that about 74% of atmospheric CH4 was consumed temperature and precipitation, soil CO2 and N2Oeffluxes in the top 4–5 cm of the mineral soil while little or no CH4 showed a clear seasonal pattern, with fluxes significantly consumption occurred in the O horizon. higher in rainy season than in dry season. CH4 consump- tions were mainly affected by soil moisture. Litter removal 4.3. Effects of land use, environmental variables and decreased CO2 and N2O effluxes at both sites in year 2 but litter removal on N2O fluxes not in year 1, and no differences were observed in CH4 fluxes between treatments. These results probably sug- Annual mean N2O efflux over 2 sampling years gested that factors other than soil moisture, temperature (mean S.E.) from pine plantation was 3.03 0.22 kg and litter exerted a larger impact on GHG fluxes and/or 1 1 N2O–N ha year , similar to the result of the forest in the that there were not enough samples for these measure- nearby Dinghushan Nature Reserve (Tang et al., 2006). The ments due to high heterogeneity of micro-site character- annual mean N2O efflux from the orchard was 8.64 istics of soils since GHG emissions varied with greatly 1 1 0.61 kg N2O–N ha year , much higher than that from spatial heterogeneity (Verchot et al., 1999, 2000). Changes pine forest. The range of N2O efflux was also higher in the of the sampling sites over the two sampling years could pine plantation that in the orchard. N2O and NO (nitric also have contributed to the large variability and oxide) were produced in soil mainly in the course of two compromised the quality of the experiment. contrasting microbial processes: nitrification (an aerobic Land use types had great effect on CO2 and N2O fluxes, process) and anaerobic denitrification (a major source of with higher rates from the orchard, an agriculture land use, N2O) (Smith et al., 2003). The fraction of the total gaseous than that from the pine plantation. It suggested that products of anaerobic denitrification that is actually emitted afforestation could be a potential mitigation strategy to to the atmosphere as N2O depends heavily on the structure reduce GHG emissions from agricultural land if the and wetness of the soil (Smith et al., 2003). Our result observed results were representative at the regional scale. suggested that the higher N2O flux from orchard was However, because the statistical analyses in this paper were correlated to higher water content in the orchard (Tables 2 based on replicate chamber measurements within the and 6). And it also was possible due to fertilizer application plantation and orchard rather than based on true replication in the orchard in previous years. of the two land use types, caution should be exercised while 134 H. Liu et al. / Agriculture, Ecosystems and Environment 124 (2008) 125–135 extrapolating results presented in this study to GHG fluxes at Li, Y.L., Peng, S.L., Zhao, P., Ren, H., Li, Z.A., 2002. A study on the soil the regional scale. carbon storage of some land use types in Heshan, Guangdong, China. J. Mount. Sci. 20 (5), 548–552. Li, Y.Q., Xu, M., Sun, O.J., Cui, W.C., 2004. Effects of root and litter

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