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Location: Plant Science Research

Title: Understanding the Variability in Soybean Nitrogen Fixation across Agroecosystems

item Schipanski, Meagan
item Drinkwater, Laura
item Russelle, Michael

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/2/2009
Publication Date: 3/1/2010
Publication URL:
Citation: Schipanski, M.E., Drinkwater, L.E., Russelle, M.P. 2010. Understanding the Variability in Soybean Nitrogen Fixation across Agroecosystems. Plant and Soil Journal. 329(1-2):379-397.

Interpretive Summary: Many contemporary farming practices rely on supplying nitrogen to the crops through commercial fertilizer or livestock manure, but it appears that these practices have led to excessive nitrogen leakage into water and air. In contrast, some farming systems rely on legumes, like soybean, clover, or alfalfa, to supply nitrogen to other crops, and these systems appear to conserve nitrogen better. Legumes can obtain nitrogen for growth from bacterial partners through the process of symbiotic nitrogen fixation. Because such legume-based systems increase the supply of nitrogen from the soil, they might cause subsequent declines in nitrogen fixation. In on-farm research on 13 farms in New York State, we found large differences in soybean nitrogen fixation -- some soybeans obtained only 48% of their nitrogen from the air while others obtained up to 77%, depending on soil nitrogen supply. However, the total amount of nitrogen fixation was determined by the yield of the plants rather than soil nitrogen supply. It also appears that legumes like soybean can improve their supply of soil nitrogen, especially on low fertility soils. This finding helps clarify the benefits of legumes in cropping systems and will help scientists learn to predict nitrogen fixation.

Technical Abstract: Conventional farming practices have uncoupled carbon (C) and nitrogen (N) cycles through the application of inorganic N fertilizers applied in plant available forms at levels that saturate the system. As a result, extensive N losses via leaching and denitrification are having significant environmental impacts. Studies of diversified, legume-based cropping systems have shown that these systems tend to increase soil organic C and N storage and achieve more favorable balances in terms of nutrient inputs and exports. However, we still do not fully understand the interactions of edaphic factors and legume N2 fixation in temperate, legume-based cash-grain agroecosystems. In this study, we used a gradient of endogenous soil N levels resulting from different field management legacies and soil textures to investigate the effects of soil organic matter dynamics and N availability on soybean (Glycine max) N2 fixation. Soybean N2 fixation was estimated on 13 grain farm fields in central New York State by the 15N natural abundance method using a non-nodulating soybean reference. A range of soil N fractions were measured to span the continuum from labile to more recalcitrant N pools. Soybean reliance on N2 fixation ranged from 48% to 77%. Soil N pools were consistently inversely correlated with % N from fixation and the correlation was statistically significant for inorganic N and occluded particulate organic matter N (r=-0.68, r=-0.65, respectively). However, the inhibitory effects of soil N availability were small in comparison to the environmental and site characteristics that determined plant net primary productivity, which was strongly correlated with total N fixed (r=0.89). Total N fixed ranged from 49 to 191 kg N/ha. In addition, we found that soil N uptake by N2-fixing soybeans relative to their non-nodulating isoline was inversely related to total soil N (r=-0.83). This suggests that N2 fixation relieved N limitation on plant growth, resulting in increased C investment to access additional soil N at low fertility sites. This study contributes to our understanding the ecology of legume N2 fixation in agroecosystems and the interactions of plant N acquisition strategies and soil organic matter dynamics. In addition, we discuss the potential application of delta 15N plant and soil signatures to study N dynamics in agroecosystems. Conducting on-farm research allows for analysis of the relative importance of different factors affecting agroecosystem processes that will aid the development of decision-support tools to improve the sustainability of nutrient management practices.