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I noticed that Andy sent the Washington Post article to the list so I
wanted to let people know that you can print copies of our paper,
entitled "Legume-based cropping systems have reduced carbon and nitrogen
losses" by going to the Nature website. (It's under the weekly feature)
The paper summarizes some of the results from the Farming Systems Trial,
a long-term experiment comparing conventional and organic maize/soybean
see the attached message below or go to www.nature.com
-- ***************************** Laurie E. Drinkwater, Ph.D. Director, US RARC
Rodale Institute 611 Siegfriedale Road Kutztown, PA 19530 voice: 610-683-1437 FAX: 610-683-8548 firstname.lastname@example.org *****************************
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Return-Path: <JLC@fien.com> Received: from smtp0-alterdial.uu.net ([126.96.36.199]) by mailhost.rodaleinst.org (Netscape Mail Server v1.1) with ESMTP id AAA5899 for <email@example.com>; Tue, 24 Nov 1998 12:07:17 -0500 Received: from cooperserver by smtp0-alterdial.uu.net with SMTP (peer crosschecked as: 1Cust3.max2.ffx1.va.alter.net [188.8.131.52]) id QQfqxf03551; Tue, 24 Nov 1998 16:54:26 GMT Message-Id: <Version.firstname.lastname@example.org> X-Sender: email@example.com (Unverified) X-Mailer: QUALCOMM Windows Eudora Pro Version 4.0 Date: Tue, 24 Nov 1998 11:22:01 -0500 To: "Organic Food and Crop Protection Issues Distribution Lists" <JLC@fien.com> From: FIEN - Jack Cooper ph 301 384 8287 fax 301 384 8340 <JLC@fien.com> Subject: Rodale Research Comparing Organic with Traditional Farming Published Cc: firstname.lastname@example.org, email@example.com Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
The following two separate articles from the November 19 issue of Nature may be of interest to you. The information is from the following www sites:
or go to the Nature home page at:
Please do not share the information with others as the articles are copyrighted.
You can contact the authors by e-mail for copies of the individual articles at:
firstname.lastname@example.org email@example.com Jack Cooper on November 24
19 November 1998=20
Nature 396, 211 - 212 (1998) =A9 Macmillan Publishers Ltd.=20
The greening of the green revolution=20
In comparison with conventional, high-intensity agricultural methods, 'organic' alternatives can improve soil fertility and have fewer detrimental effects on the environment. These alternatives can also produce equivalent crop yields to conventional methods.
It is not clear which are greater -- the successes of modern high-intensity agriculture, or its shortcomings. The successes are immense. Because of the green revolution, agriculture has met the food needs of most of the world's population even as the population doubled during the past four decades. But there has been a price to pay, and it includes contamination of groundwaters, release of greenhouse gases, loss of crop genetic diversity and eutrophication of rivers, streams, lakes and coastal marine ecosystems (contamination by organic and inorganic nutrients that cause oxygen depletion, spread of toxic species and changes in the structure of aquatic food webs)1,2. It is unclear whether high-intensity agriculture can be sustained, because of the loss of soil fertility, the erosion of soil, the increased incidence of crop and livestock diseases, and the high energy and chemical inputs associated with it1,3. The search is on for practices that can provide sustainable yields, preferably comparable to those of high-intensity agriculture but with fewer environmental costs1.
For millennia, farmers countered the loss of soil fertility caused by agriculture (Fig. 1) by manuring fields, by alternating crops that increase soil fertility (such as legumes, which 'fix' atmospheric nitrogen into organic compounds and so add nitrogen-containing compounds to the soil) with other crops, and by abandoning fields and allowing them to be taken over gradually by natural vegetation (succession). This changed with the advent of the green revolution.
Figure 1 Typical effects of different agricultural practices on the total organic carbon or nitrogen content of soil. Full legend
High resolution image and legend (10k)
A hallmark of high-intensity agriculture is its dependence on pesticides and chemical fertilizers, especially those containing nitrogen. Since 1960 the worldwide rate of application of nitrogen fertilizers has increased by seven times2, and now exceeds 7=D7107 tonnes of nitrogen per year. Inputs from humans now equal all natural inputs to the nitrogen cycle and are seriously affecting terrestrial, freshwater and marine ecosystems2, because half to two-thirds of nitrogen fertilizers enter these non-agricultural ecosystems.=20
On page 262 of this issue4, Drinkwater, Wagoner and Sarrantonio report two alternative practices for growing maize that maintain yields while increasing soil fertility and decreasing losses of nitrogen by leaching. This advance is not based on a miracle of technology but is a lesson from agriculture's past that may presage its future.
In Drinkwater and colleagues' conventional, high-intensity system, pesticides and mineral nitrogen fertilizer were applied to a maize/soybean crop rotation just as on typical farms. Two 'organic' alternatives represented partial returns to traditional agriculture, and neither synthetic fertilizers nor pesticides were used. One of these alternatives was a manure-based system in which grasses and legumes, grown as part of a high-diversity crop rotation, were fed to cattle. The resulting manure provided nitrogen for periodic maize production. The other system did not include livestock; instead, nitrogen fixed by a variety of legumes was incorporated into soil as the source of nitrogen for maize.
Amazingly, ten-year-average maize yields differed by less than 1% among the three cropping systems, which Drinkwater et al. say were nearly equally profitable. The manure system, though, had significant advantages. Soil organic matter and nitrogen content -- measures of soil fertility -- increased markedly in the manure system (and, to a lesser degree, in the legume system), but were unchanged or declined in the conventional system. Moreover, the conventional system had greater environmental impacts -- 60% more nitrate was leached into groundwater over a five-year period than in the manure or legume systems.
Why were the organic methods superior to conventional, high-intensity agriculture? The answer is not yet known, but two possibilities stand out. First, when fertilizing, timing is crucial5. The nitrogen pulse from a single application of mineral fertilizer can cause soil nitrate concentrations to greatly exceed plant needs. The unconsumed nutrients are susceptible to loss by leaching and denitrification. In contrast, the organic methods supply nitrogen in organic forms that gradually release mineral nitrogen, perhaps better synchronizing nutrient availability with plant needs.
Second, although equivalent amounts of nitrogen and organic carbon were added to the soil in all three systems, the manure system included a higher proportion and greater diversity of recalcitrant (that is, slowly biodegradable) organic compounds than the conventional system. This may have caused carbon and nitrogen to accumulate in the manure system, minimizing leaching losses. Indeed, models of soil carbon and nitrogen dynamics predict such accumulation when fields are manured6,7 (Fig. 1).
Drinkwater and colleagues' results may seem astounding, or even suspect, given the widespread use of chemical fertilizers. They are not. In the Broadbalk experiment (Fig. 2, overleaf), at the Rothamsted Experimental Station in the United Kingdom, which has been running for more than 150 years, wheat yields have averaged 3.45 tonnes per hectare on manured plots compared with 3.40 tonnes per hectare on plots receiving complete nitrogen, phosphorus and potassium (NPK) fertilizer8. Moreover, soil organic matter and soil total nitrogen levels increased by about 120% over 150 years in the manured plots (Fig. 1), but by only about 20% in the NPK plots7,9. Such carbon stores might represent an underappreciated sink for global carbon.=20
Figure 2 Field work: experiments on fertilizer regimes have run at Rothamsted since 1843. Full legend
High resolution image and legend (211k)
The intensification of agriculture has broken what was once the tight, local recycling of nutrients on individual farms. Indeed, the green revolution and the large-scale livestock operations that have come with it are reminiscent of the early stages of the industrial revolution, when inefficient factories polluted without restriction. The US Environmental Protection Agency estimates that, in the United States alone, livestock operations generate about 109 tonnes of manure per year, much of it in large-scale operations in which up to a million or more animals are housed in close quarters. These concentrated sources of manure are often too far from farms to be economically transported to them, or are applied at inappropriately high rates or at incorrect times, or are released into waterways without removing nitrogen and phosphorus. This has created an open nitrogen cycle that is rapidly degrading many other ecosystems2. Sustainable and productive ecosystems have tight internal cycling of nutrients, a lesson that agriculture must relearn.=20
The results of Drinkwater and colleagues4 are a step in the right direction. What may lead to further progress? The green revolution turned developments in crop genetics, inexpensive pesticides and fertilizers, and mechanization into greater yields. Further advances, such as precision agriculture, in which fertilizer application rates and timing are adjusted differentially across a field to meet crop needs, will increase agricultural efficiency and decrease adverse effects on the environment. However, a greener revolution is also needed -- a revolution that incorporates accumulated knowledge of ecological processes and feedbacks, disease dynamics, soil processes and microbial ecology. Experiments such as those of Drinkwater et al. need to be combined with studies of both the mechanisms controlling soil organic matter and nitrogen dynamics6,7,9, and the dynamics of crop nutritional needs.
The principles of ecology, epidemiology, evolution, microbiology and soil science operate in agroecosystems as well as in natural ecosystems. Although the owners of the businesses were probably shocked, I doubt if epidemiologists were surprised that Hong Kong chicken operations, housing up to a million genetically similar chickens, were susceptible to a rapid and devastating outbreak of disease last year. When those running massive livestock operations realize that chronic disease and catastrophic epidemics are the expected result of high densities and low diversity, and when society restricts the release of pollutants from such operations, it may again be profitable for individual farms, or neighbourhood consortia, to have mixed cropping and livestock operations tied together in a system that gives an efficient, sustainable, locally closed nitrogen cycle.
No other activity has transformed humanity, and the Earth, as much as agriculture10, but the environmental effects of high-intensity farming increasingly haunt us. In a small world awash with the waste products of humanity, there is a great need to find new approaches to agriculture.
David Tilman is in the Department of Ecology, Evolution, and Behavior, University of Minnesota, St Paul, Minnesota 55108, USA. e-mail: firstname.lastname@example.org
Matson, P. A., Parton, W. J., Power, A. G. & Swift, M. J. Science 277, 504-509 (1997). Links=20 Vitousek, P. M. et al. Ecol. Appl. 7, 737-750 (1997). Links=20 Pimentel, D. et al. Science 267, 1117-1123 (1995). Links=20 Drinkwater, L. E., Wagoner, P. & Sarrantonio, M. Nature 396, 262-265 (1998). Links=20 Matson, P. A., Naylor, R. & Ortiz-Monasterio, I. Science 280, 112-115 (1998). Links=20 Parton, W. J. & Rasmussen, P. E. Soil Sci. Soc. Am. J. 58, 530-536 (1994). Links=20
Jenkinson, D. S., Bradbury, N. J. & Coleman, K. in Long-term Experiments in Agricultural and Ecological Sciences (eds Leigh, R. A. & Johnston, A. E.) 117-138 (CAB Int., Wallingford, UK, 1994). Links=20
Johnston, A. E. in Long-term Experiments in Agricultural and Ecological Sciences (eds Leigh, R. A. & Johnston, A. E.) 9-37 (CAB Int., Wallingford, UK, 1994). Links=20
Powlson, D. S. in Long-term Experiments in Agricultural and Ecological Sciences (eds Leigh, R. A. & Johnston, A. E.) 97-115 (CAB Int., Wallingford, UK, 1994). Links=20
Diamond, J. M. Guns, Germs, and Steel: The Fates of Human Societies (Norton, New York, 1997). Links=20
Nature =A9 Macmillan Publishers Ltd 1998 Registered No. 785998 England.=20
19 November 1998=20
Nature 396, 262 - 265 (1998) =A9 Macmillan Publishers Ltd.=20
Legume-based cropping systems have reduced carbon and nitrogen losses=20
L. E. DRINKWATER, P. WAGONER & M. SARRANTONIO*=20
Rodale Institute, 611 Siegfriedale Road, Kutztown, Pennsylvania 19530, USA
* Present address: Sustainable Systems Program, Slippery Rock University, Slippery Rock, Pennsylvania 16057, USA.
In agricultural systems, optimization of carbon and nitrogen cycling through soil organic matter can improve soil fertility and yields while reducing negative environmental impact. A basic tenet that has guided the management of soil organic matter for decades has been that equilibrium levels of carbon and nitrogen are controlled by their net input and that qualitative differences in these inputs are relatively unimportant1,2,3. This contrasts with natural ecosystems in which there are significant effects of species composition and litter quality on carbon and nitrogen cycling4,5. Here we report the net balances of carbon and nitrogen from a 15-year study in which three distinct maize/soybean agroecosystems are compared. Quantitative differences in net primary productivity and nitrogen balance across agroecosystems do not account for the observed changes in soil carbon and nitrogen. We suggest that the use of low carbon-to-nitrogen organic residues to maintain soil fertility, combined with greater temporal diversity in cropping sequences, significantly increases the retention of soil carbon and nitrogen, which has important implications for regional and global carbon and nitrogen budgets, sustained production, and environmental quality.=20
We studied carbon and nitrogen balances in two legume-based and one conventional, fertilizer-driven agroecosystem. The conventional system consisted of a maize/soybean rotation; a mineral nitrogen fertilizer was applied before maize was planted and pesticides were used as needed. The other two cropping systems depended on legumes for nitrogen fixation and were managed on the basis of 'organic' (US) or 'ecological' (Europe) strategies, avoiding the use of synthetic fertilizers and pesticides6. One system simulated a beef operation in which crop biomass (legumes and grasses) was fed to beef cattle and the manure was subsequently returned to the field as the primary nitrogen source for maize (MNR system). The other system received nitrogen directly from legumes through incorporation of leguminous biomass before maize planting (LEG system). Maize and soybeans were not present as frequently in these systems as in the conventional system, because small grains and several other legumes were also included in the rotation. Ten-year averages for 1986-95 maize yields were 7,140, 7,100 and 7,170 kg ha-1 in the MNR, LEG and conventional systems, respectively, and were not significantly different (analysis of variance ANOVA, P > 0.5). For the past ten years of the experiment, economic profitability from the three systems has been comparable7.=20
As a result of these distinct management strategies, there were significant quantitative and qualitative differences in organic residue inputs and in soil carbon sequestration. The conventional system had greater mean cumulative above-ground net primary productivity (ANPP, Table 1) and returned more crop residues to the soil than did both of the legume-based systems. The MNR system had the greatest harvest intensity; only 36% ANPP was returned to the soil as crop residues. Total carbon returned to the soil in the MNR and conventional systems, however, was not significantly different, because of steer manure additions in the MNR system (Table 1). The quantity of carbon inputs was not the major factor affecting soil carbon storage in these cropping systems. Even though the MNR and conventional systems received equal amounts of carbon, only the MNR system showed a significant increase in carbon stored in soil (Table 1). The LEG system, with lower average carbon inputs from above-ground sources, also showed an increase in soil carbon.=20
In contrast to the conventional system, which received only senescent-crop residues (Table 1), the two legume-based systems received relatively diverse residues that differed in terms of biochemical composition (the residues were from senescent crops, leguminous biomass and/or steer manure). Studies of litter-quality effects using agricultural residues have produced inconsistent results1,2,3,8,9. It is likely that the increased carbon storage in the MNR system is partially due to the return of steer manure to the field. Compared with senescent-crop residues, a larger proportion of manure-derived carbon is retained in soil, probably because manure is already partly decomposed and contains a larger proportion of chemically recalcitrant organic compounds8,9. On the other hand these studies did not find significant effects of types of plant species on long-term carbon equilibrium1,8,9.=20
We studied the potential role of plant-species differences on soil carbon storage, using variations in the natural abundance of 13C associated with photosynthetic pathways to estimate the relative contribution of C4 and C3 plants to soil organic matter (SOM). Maize, the only C4 crop present, accounted for 74%, 48% and 22% of the returned residues in the conventional, LEG and MNR systems, respectively. In the conventional system, maize-derived carbon still replaces the original soil carbon deposited by the C3 temperate forests that preceded agriculture in this region. In this case, net soil carbon levels did not change because the loss of C3-derived carbon was nearly equivalent to the gain of C4-derived carbon (Fig. 1). In contrast, the net gains in soil carbon seen in the LEG and MNR systems were due to significant increases in C3-derived carbon. Levels of soil carbon derived from C4 plants did not change in the MNR and LEG systems. In the LEG system levels of C3-derived carbon were disproportionately high, accounting for 88% of the net increase in soil carbon although only half of the residues returned were from C3 plants. Replacement of C4-derived soil carbon in pools with turnover times of less than 20 years, which accounts for about 5% of the total soil carbon10, cannot explain this discrepancy. These changes in the natural abundance of 13C in SOM in the LEG system indicate that differences in plant-species composition have contributed to differential retention of soil carbon. Plant species can affect carbon equilibrium through differences in below-ground net primary productivity (NPP)11, the timing and level of root turnover/exudates12, litter quality4, tendencies to foster the formation of soil aggregates13, and changes in microbial community structure and function14,15.=20
Figure 1 Soil carbon levels in 1981 (left-hand bars) and 1995 (right-hand bars); means s.e.m. are shown. Full legend
High resolution image and legend (17k)
Qualitative differences in nitrogen inputs also had a major influence on nitrogen retention in these agroecosystems. Nitrogen losses due to leaching in 1991-95 were comparable in the LEG and MNR systems, averaging 13 kg nitrogen ha-1 yr-1, but were about 50% higher in the conventional system, averaging 20 kg ha-1 yr-1, (ANOVA, P =3D 0.06; Fig. 2). Seasonal effects in all cropping systems were similar, with the greatest losses occurring during the late-fall to early-spring months when mineralization tends to exceed crop demand. Leaching losses were greatest from late fall of 1991 to spring of 1993 compared with the later half of the rotation cycle, particularly in the conventional system.=20
Figure 2 Cumulative nitrate leaching during 1991 to 1995. Full legend
High resolution image and legend (8k)
Cumulative nitrogen additions were similar in the conventional and MNR systems, as were nitrogen exports from non-leguminous crops (Fig. 3a, b). Over the course of 15 years, nitrogen inputs from soil amendments have exceeded exports by crops by a total of 520 kg ha-1 in the conventional and 540 kg ha-1 in the MNR systems (Fig. 3c). Despite these similarities in net balance, there were significant differences in soil nitrogen storage. Most of the surplus nitrogen received by the MNR system over the 15 years can be accounted for by the significant increase in soil nitrogen from 1981 to 1995 (Fig. 3d), whereas in the conventional system soil nitrogen levels have decreased since 1981 (Fig. 3d).=20
Figure 3 Comparison of cumulative nitrogen inputs and exports and changes in soil nitrogen storage after 15 years. Full legend
High resolution image and legend (16k)
Nitrogen inputs into the LEG system are more difficult to quantify because nitrogen fixed by the green manure was the major nitrogen input. However, we have estimated the maximum nitrogen input from the green manure over 15 years to be 840 kg nitrogen ha-1. If the proportion of nitrogen fixed ranged from 75% to 100%, nitrogen inputs from the green manure would have been 630-840 kg ha-1 (Fig. 3a). Non-leguminous exports were not significantly different from those in the conventional system, but were somewhat lower than those in the MNR system (protected Scheffe's, P < 0.05; Fig. 3b). Soil nitrogen levels in this system have not changed significantly (Fig. 3d).=20
The nitrogen unaccounted for in our balance calculations was +240, 100 and +1,020 kg ha-1 in the MNR, LEG and conventional systems, respectively, and probably reflects differences in gaseous losses and nitrogen fixation by soybeans not included in our calculations. Although we have no measurements of gaseous losses, we can estimate the potential impacts of soybeans on nitrogen balance on the basis of data from the experiment. Nitrogen balances suggest that nitrogen fixation was probably lower in conventional-system soybeans and greater in LEG-system soybeans. Calculations for possible nitrogen-fixation scenarios in the LEG and conventional systems show that even large differences in soybean nitrogen-fixation rates between systems did not alter the basic trends in nitrogen balance. For example, if nitrogen fixed by soybeans were 75% of total soybean nitrogen in the LEG system, the net nitrogen input from senescent roots and shoots would have been only 60-140 kg ha-1 over 15 years. Likewise, nitrogen extraction by soybeans in the conventional system can account for only part of the missing 1,020 kg of nitrogen (surplus + change in soil nitrogen); that is, a nitrogen-fixation rate of 40% of soybean total nitrogen would result in the extraction of 280-350 kg ha-1 over 15 years, a fairly typical outcome for highly determinant soybeans in maize rotations16.=20
These results for nitrogen parallel our findings for carbon, indicating that quantitative differences in nitrogen balance were not the major factor affecting soil nitrogen retention. Instead, qualitative differences in the form of nitrogen inputs and subsequent effects on internal nitrogen cycling had a significant impact on long-term soil nitrogen retention. Detailed microplot studies using 15N as a tracer showed that there are differences in the partitioning of nitrogen from organic versus mineral sources, with more legume- derived nitrogen than fertilizer-derived nitrogen immobilized in microbial biomass and SOM17,18. If immobilization is lower in the conventional system compared with in the other two systems, this may explain the greater leaching of NO-3 that we observed in this system. Differences in plant-species composition may contribute to reducing NO-3 leaching by scavenging soil nitrogen during periods in which summer cash crops, such as maize and soybeans, are not active19.=20
Our results show that even in these intensively managed agroecosystems, plant-species composition and litter quality influence SOM turnover markedly. Increases in SOM in the MNR and LEG systems were highly significant in terms of ecosystem function and soil quality. Greater retention of both carbon and nitrogen suggests that use of low carbon-to-nitrogen residues to maintain soil fertility combined with increased temporal diversity restores the biological linkage between carbon and nitrogen cycling in these systems and could lead to improved global carbon and nitrogen balances. Application of these practices in the major maize/soybean growing region in the USA would increase soil carbon sequestration by 0.13-0.30 1014 g yr-1. This is equal to 1-2% of the estimated annual carbon released into the atmosphere from fossil fuel combustion in the USA20 (1.4 1015 g carbon yr-1, 1994) and is a significant contribution considering that the USA has agreed to reduce average CO2 emissions to 7% below 1990 levels by 2008-2012 as part of the Kyoto Protocol. In addition, CO2 emissions from the two legume-based systems are lower than emissions from the conventional system because of a 50% reduction in energy use21. The potential effects on the nitrogen cycle are much greater in magnitude because the fixed nitrogen used in agricultural activities is responsible for a 60% increase in global levels of biologically active nitrogen22. Reduced nitrogen losses combined with increased soil nitrogen storage will lead to reductions in the amount of nitrogen that must be applied to maintain yields.=20
The experiment covered 6 ha and consisted of a randomized, complete block design. Details of experimental design and farming practices are described elsewhere23. Carbon contents of organic residues are calculated from plant biomass data collected from 1981-1995 assuming a carbon content of 42% on a dry weight basis.=20
Soil analyses. Composite soil samples collected in 1981 and 1995 were analysed for total carbon and nitrogen with the Leco CN-2000 analyser (Leco Corporation). Natural abundance of 13C was determined by combustion of triplicate samples with a Europa Scientific carbon-nitrogen analyser connected to a Europa Scientific Tracermass mass spectrometer. To estimate the original 13C natural abundance before the introduction of C4 plants, we collected a composite soil sample along two transects from a forested, never-farmed site located 0.5 km from the experiment. The proportion of carbon derived from corn residues, C4%, was calculated for 1981 and 1995 as C4% =3D (m - f)/(mr - f) 100, where m =3D 13C content of soil after maize cultivation; r =3D -25.58 the 13C content of soil under th= e nearby native mixed hardwood forest; and mr =3D -13.1 the 13C content of maize residues24.=20
Nitrogen budget. Nitrogen inputs from leguminous green manures were calculated by using above-ground biomass nitrogen content data and adjusting for below-ground contributions using root biomass data and nitrogen content from the experiment in 1997. Values for total nitrogen fixation in these plants were taken from published studies to estimate the minimum proportion of nitrogen fixed16,25. Soybeans, which were present in all three cropping systems, were not included in nitrogen-budget calculations because of the potential for variability in nitrogen fixation by soybeans16. However, we used our data on soybean yields, residue returns and nitrogen contents to estimate potential impacts on nitrogen balance. Usually, no more than 50% of total soybean nitrogen is derived from nitrogen fixation and generally two-thirds of the total nitrogen is exported from the beans3,16. Nitrogen fixation by free-living bacteria, which represents a minor contribution to nitrogen inputs in agricultural systems using tillage, was assumed to be 5 kg nitrogen ha-1 yr-1 (ref. 26). In earlier work we did not find significant differences in the potential for nitrogen fixation by free-living organisms in these cropping systems27. We estimated nitrogen inputs from atmospheric deposition to be 15 kg nitrogen ha-1 yr-1 on the basis of data for northeastern USA28. Estimates of nitrate leaching for the 15-year period were based on five years of data collected in this experiment during 1991-95 from intact-core below-ground lysimeters installed in 4 replicates 3 entry points, for a total of 36 lysimeters (each 0.45 m2 in area)29.=20
Received 8 January;accepted 2 September 1998.=20
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Acknowledgements. We thank M. Cavigelli, J. Easter, P. Groffman, D. Jenkinson, P. Matson and S. Snapp for comments on the manuscript; J. Duxbury for discussions of 13C natural abundance methodology; and E. A. Paul for funding and analytical support of lysimeters and the NO-3 leaching component. This work was funded in part by USDA-ARS.
Correspondence and requests for materials should be addressed to L.E.D. (e-mail: email@example.com).
Nature =A9 Macmillan Publishers Ltd 1998 Registered No. 785998 England.=20
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