Recycling Nitrogen and Sulfur in Grass-Clover Pastures 4.tj AGRICULTURAL EXPERIMENT STATION OREGON STATE UNIVERSITY CORVALLIS STATION BULLETIN 610 JUNE 1972 Contents Abstract 3 Introduction 3 The Nitrogen Cycle 3 The Sulfur Cycle 8 Summary 11 LiteratureCited 12 AUTHORS: M. D. Dawson is a professor of soils science and W. S. McGuire is a professor of agronomy, Oregon State University. ACKNOWLEDGMENTS: The authors are indebted to Viroch Impithuksa for conducting the carbon-nitrogen-phosphorus-sulfur (C:N:P:S) analyses and to J. L. Young for invaluable assistance in writing the manuscript. Recycling Nitrogen and Sulfur in Grass-Clover Pastures M. D. DAWSOTJ and W. S. McGuii Abstract Indeed, the soil-plant-animal chain is a fascinating intra-system where the Improved grass-clover pastures uti-nitrogen and sulfur cycles have practi- lized under high stockingsystems cal significance. epitomize conservation management at The management practicescom- its best. Under intensive grazing andpared are (1) unimproved, indigenous in spite of nitrogen (N) or sulfur (S) grasses, (2) fertilized grass-clover cut losses through leaching, volatilization, for hay, and (3) fertilized grass-clover or sales of meat and wool from the intensively grazed. The purpose of this farm, good management permits sym- bulletin is to review certain features bioticfixationofnitrogen and re-of soil-plant-animal interrelationships cycling of N and S in amounts needed as they influence soil nitrogen and sul- for top production. In the comparisons fur cycles under different management of management practicesinvolvingpractices in grass-clover pastures. unimproved indigenous grasses with (1) fertilized grass-clover cut for hay and (2) fertilized grass-clover inten- The Nitrogen Cycle sivelygrazed,thisbulletin reviews In western Oregon, annual yields of certainfeaturesof soil-plant-animal6,000 pounds and 12,000 pounds of interrelationships as they influence soildry matter per acre are common on nitrogen and sulfur cycles. subterraneancloverandirrigated grass-whiteclover pastures,respec- tively. The 6,000 pounds of dry matter Introduction from healthy subterranean clover pas- Many acres of western Oregon landture should contain about 3 percent not ideally suited to cultivation cannitrogen. At least 180 pounds nitrogen produce much meat and wool by wayper acre would be needed to produce of intensively grazed pastures. High this 6,000 pounds of dry matter. Twice stocking rates on these improved grass-this amount of nitrogen would be clover pastures are a means by whichneeded to produce the 12,000 pounds man can economically provide goodof dry matter on irrigated grass-white quality, high protein livestock feed.clover pasture. But quality forage requires consider- The average total soil nitrogen con- able nitrogen, phosphorous, and sulfate tent under normal pastures (Table 1) in balance. Few crops have a higher demand for these nutrients than aTable 1.Average percentage total soil vigorously growing grass-clover pas- nutrients' ture. Percent It is of considerable economic and Nutrient environmental interest to examine the Total nitrogen 0.24 cyclical changes of the comparativelyTotal sulfur 0.02 mobile nutrientsnitrogen and sulfurTotal organic phosphorus 0.03 under variously managed pastures. 'From 12 Oregon grass-clover pastures. 3 N RETAINED BY STOCK VOLATILE STOCK GRAZED N A GRASS CLOVER mdrgromd SYMBIOTIC N FOCER URINE flnsference / ATMOSPHERE N / N NITRATE DEN1TRIFIED AND NITRATE LEACHED \I N FROM PLrr ROOTS SOIL ORGANIC N Figure 1.The nitrogen cycle on grazed grass-clover pasture. Adapted from T. W. Walker (12). is low, about 0.20 percent. In an acrein nodulated roots of the clover plants. furrow s]ice of soil (2 x 106 lbs.) that Numerousworkershavetriedto would amount to 4,000 pounds ofmeasure the amounts of nitrogen fixed nitrogen, but most of this is bound in by clovers annually. Estimated net ad- thesoilin complex organic forms.ditions vary from nil (where the hay Grasses and clover obtain some of thisis removed) to 10 pounds nitrogen per soil-bound nitrogen as nitrate or am-acre (where the legume is sparse) to monium after the decay and minerali- 100 up to more than 400 pounds nitro- zation of soil organic matter. However, gen per acre (where effective legume there is evidence that only about 1.25is plentiful). The amount depends on percent (11)1 of the total soil nitrogen such factors as species of legume, a complex ingrass-clover pasturesis proven effective rhizobia strain, and a mineralized to plant-available forms host of environmental conditions (10). annually. This would mean at best 50 The amount of nitrogen fixed and pounds of nitrogen could be min-available, either for companion grass eralized from 4,000 pounds in an acreby undergroundtransferencefrom furrow slice annually, or a total deficitclover roots or for the clover plant it- of 130 pounds (180-50) of plant-avail- self, depends on effective nodulation able nitrogen per acre. which is only possible with proven ef- Figure1, adapted from Walker'sfective rhizobium strains(Table 2). nitrogen cycle model (12), shows theThere is evidence that "native" strains nitrate mineralized from decaying soilof rhizobia are in many cases only organic matter is subject to uptake by partially effective in N fixation for the pasture, mainly by the grass com- clovers grown in Oregon. ponent. Also evident is an arrow indi- For subterranean clover, an aver- cating symbioticfixationof atmos-age of 150 pounds nitrogen per acre pheric nitrogen by the rhizobia livingis fixed symbiotically per year. That is more than enough to make up the 'Numbers in parentheses refer to Liter- 130-pound deficit of nitrogen previ- ature Cited, page 12. ously noted, except that, firstly, much 4 Table 2.Effect of inoculation with a othernutrientstoinsure optimum proven rhizobium strain on New Zealand symbiotic nitrogen fixation. Table 4 white c1over data illustrate the effect of applied sul- fur and molybdenum on clover yield Inoculated Uninoculated and reflect the role these nutrients Avg. yield Avg. Avg. yield Avg. have upon thesymbioticnitrogen (dry plant (dry plant fixation efficiency. Such responses have matter) N matter) N been observed frequently in Oregon. lbs/A % lbs/A % Where grasses and clovers are growing 11,574 3.17 7,903 2.18 together, grasses utilize almost all the mineral N available (11). Recent research showsthatsoil of this fixed nitrogen is not immedi-moisture stress may be as much a ately available to the companion grasslimiting factor on the ability of clover and, secondly,soil conditions often to symbiotically fix atmospheric nitro- preclude the clover's ability to fix nitro- gen as are deficiencies in plant nu- gen efficiently. Ineffectively nodulatedtrients. Kuo (6) found that nitrogen plants, due often to poor sowing tech-fixation by a legume is reduced as the riiques, produce clover which is lowsoil dries out (Table 5). Together with in plant nitrogen (Table 3). Data inleaching losses of nitrates during win- Table 3indicatethat conventional ter and volatile losses of nitrogen from inoculation was often ineffective. Tothe soil, such less-than-optimum condi- insure effective nodulation with thetions for nitrogen fixation increase the applied strain,it was necessary to practical difficulties of obtaining suffi- plant seeds with a lime-superphos-cient nitrogen for the yield of 6,000 plite mixture. pounds dry matter per acre in a grass- Once established, a legume must beclover pasture. supplied with adequate phosphorus, An ideal way to restore balance to sulfur,molybdenum,andpossibly the nitrogen cycle (Figure 1) is to use Table 3.Percent effective nodulatjoof subclover and plant nitrogen content Percent effective nodulation four weeks after planting Average Washington plant Treatment Polk Co. Co. Coos Co. nitrogen (4 Uni noculated 14 2 3 1.29 Inoculated 14 2 44 Inoculat d and ''line- super mix' 72 96 79 3.54 Equal ,uixture of 20 percent superphosphate and lime. Table 4.Mean subterranean clover yields and nitrogen content as influenced by applied sulfur and molybdenum Total Clover plant Treatment Diy matter nitrogen nitrogen lbs/A lbs/A J) 3,420 2.06 70 PS 5,220 2.78 145 PSMo 5,982 3.17 190 5 Table 5.Rates of nitrogen fixation in a legume as influenced by moisture stress at 50 degrees F' Soil water stress Nitrogen fixaon (bars) Soil status ( mg of N per day) 0.35 Field capacity 0.76 1.50 Moist 0.30 2.50 Moderately dr 0.23 'From unpublished M.S. thesis by T. Kuo (6). Table 6.Nitrogen uptake from grass and grass-plus-clover pastures' Grass-clover pasture Management Grass alone Grass N Clover N Total lbs/A lbs/A lbs/A lbs/A Hay crop taken 50 188 346 534 Grazed 75 379 271 650 J. Melville and P. D. Sears, (7). the grazing animal. The data in Tableapparent when ryegrass was grown 6 (taken from New Zealand) illustrate in a field fertilized with superphos- the role of the grazing animal in thephate (300 lbs.of 20% superphos- nitrogen cycle. Where a hay crop isphate applied annually) that had been taken,underground transferenceof intensively grazed (4 sheep per acre) nitrogenfrom cloverhas providedfor the past 12 years. Furthermore, grass in association with clover withthe yield and nitrogen content of rye- 138 pounds more nitrogen per acregrass on thissoil were significantly than an all-grass pasture. This was a higher than of ryegrass grown on the small effect compared with the grazedsame soil which had virtually no graz- grass-clover pasture, where the grass ing during this period. The accumu- uptake of nitrogen amounted to 379lated soil organic nitrogen on the in- pounds per acre. tensively grazed grass-clover pasture A greater proportion of the nitrogen apparently is mineralized and cycled originally fixed by clover appears in at a rate sufficient to provide optimum the grass where urine is returned andnitrogen nutrition for the grass. Under serves as a nitrogen fertilizer. Indeed, such conditions,the nitrogencycle at a stocking rate of three or four ewes appears balanced in such a way that per acre, dung and urine excreta would moisture and sufficient solar energy likely return an equivalent of at least alone drive the system near optimum.