The effect of management history on plant uptake of nitrogen from a legume cover crop: A 15N tracer study

Background/Question/Methods

A fundamental challenge in agriculture is to meet the nitrogen demand of crops without causing water pollution and greenhouse gas emissions from excess nitrogen. Legume cover crops can reduce the need for chemical fertilizers, providing nitrogen in a stable form that is less prone to becoming a pollutant. The goal of this work was to test the effect of management history on the use of field pea as a nitrogen source for wheat. We collected Honeoye soils from two adjacent sites with contrasting management histories in Penn Yan, New York, USA. The organically managed site (ORG) uses legumes and manure to provide nitrogen. The adjacent field has been conventionally managed (CNV), receiving greater total nitrogen inputs from chemical fertilizer. Our hypothesis is that wheat grown in ORG soils will use more legume nitrogen than wheat grown in CNV soils because in contrast to the CNV system, the ORG soil ecosystem relies on the microbial community to mineralize organic nitrogen. We conducted a growth chamber experiment in which we grew wheat in soil from the two sites. The only fertility provided was ¹⁵N-enriched pea biomass. Wheat was harvested at maturity to measure plant biomass, grain biomass, and grain ¹⁵N content. 

Results/Conclusions

Wheat grain yield in ORG soil was twice the yield from CNV soil (12 g grain per pot versus 6 g) and total plant biomass was 83% greater in ORG soil (17 g total plant per pot versus 9 g). We expect the ¹⁵N data will confirm that plants in ORG soil accessed more legume cover crop nitrogen than plants in CNV soil. These results suggest wheat in conventionally managed soil suffered from nitrogen limitation despite having an adequate supply of organic nitrogen, and that management history shapes the soil microbial community with consequences for agricultural productivity and the ability of wheat to access organic nitrogen sources.