Compost Legacy Down‐Regulates Biological Nitrogen Fixation in a Long‐Term Field Experiment

Abstract

Core Ideas Compost‐based management enhanced soil organic matter in a Michigan Alfisol. Nitrogen fixation rates in soybean and red clover varied with management history. Weather markedly influenced the quantity of N fixed in three crop systems. Soil labile pool influence on N fixation can act as an internal regulator. Biological nitrogen fixation (BNF) is a fundamental process relied on in agriculture, yet few field studies have examined regulation through soil inorganic N feedbacks or considered seasonal effects. In a Michigan long‐term field experiment we examined soil labile C and N pools and impact on BNF in two species, over multiple years. The 15N natural abundance method was used to quantify BNF, with nodulated and non‐nodulated soybean [Glycine max (L.) Merr.] isolines and red clover (Trifolium pratense L.). Soil organic C status of plots was consistent with a gradient established through historical inputs: 0.8% (fertilizer management) to 0.9% (+ cover crop) to 1.2% (+ compost + cover crop). The fraction of nitrogen derived from atmosphere (fNdfa) in red clover and soybean over 3 yr was 61 to 58% with compost management, and 71 to 72% with fertilizer. This represented a downregulation of 19% (red clover) and 15% (soybean) in compost and cover crop managed plots, relative to inorganic fertilizer. In addition to management effects, we found that weather markedly influenced the total amount of N fixation over the 3 yr of the study. A mesic season supported vigorous soybean growth and 195 kg N ha−1 BNF, compared to 95 and 90 kg N ha−1 BNF in a dry and an excessively wet season. This study found that compost‐based management increased pools of labile C and N that internally downregulated BNF, while enhancing soybean yield compared to conventional management.

Nitrification is a minor source of nitrous oxide (N2O) in an agricultural landscape and declines with increasing management intensity

“…Red clover was frost‐seeded into wheat in March, lay dormant over winter, and was terminated just prior to planting corn the following spring. Over this period, it fixes ~35–53 kg N ha −1 (Snapp et al, 2017 ). Both red clover and ryegrass scavenge soil N otherwise leached or denitrified.…”

The Biology of Legumes and Their Agronomic, Economic, and Social Impact

“…Biological N fixation by legumes (e.g., faba bean, lentil, pea, chickpea, alfalfa, red clover etc.) ranges from 21 to 389 kg ha −1 (Table 1) (Cazzato et al 2012;Nimmo et al 2013;Büchi et al 2015;Hossain et al 2016;Snapp et al 2017;Akter et al 2018;da Silva Júnior et al 2018;Dhamala et al 2018;Ntatsi et al 2018;Pampana et al 2018;Ntatsi et al 2019).…”

Crops, Nitrogen, Water: Are Legumes Friend, Foe, or Misunderstood Ally?

“…In a study of a range of herbaceous legumes that grow in the Bay area of California, BNF was strongly downregulated by moderate additions of N fertilizer [41]. In a study using manures as a source of organic N, BNF was downregulated relative to no N treatments [42]. The precise mechanism of regulation within root nodules remains an active area of research.…”