Abstract
Sustainable nitrogen (N) management in agroecosystems is crucial for supporting crop production and reducing deleterious N losses. Biochar application with N-fixing legumes offers promise for increasing soil N retention and input. Strategic, low application rates (112 kg ha−1) of pine and coconut feedstock biochars were tested in an established alfalfa (Medicago sativa) field. Soil inorganic N and plant growth, N concentrations, and δ15N were monitored over a growing season to follow mineral N availability, and plant N uptake and sourcing. Microbial and gene abundance and enzyme activity were measured to assess the potential for N cycling processes to occur. Biochar application had minimal effects on measured parameters. However, significant temporal dynamics in N cycling and correlations between alfalfa δ15N and soil N availability indicate differing plant N sourcing over time. Our findings indicate that low application rates of biochar in established alfalfa fields do not significantly affect N cycling, and that managing alfalfa to maximize N fixation, for example by intercropping, may be a better solution to increase N stocks and retention in this system. To determine when biochar can be beneficial for alfalfa N cycling, we need additional research to assess various economically-feasible biochar application rates at different alfalfa growth stages.
Highlights
A key facet of soil health and sustainable agriculture is supporting the ability of soils to retain and cycle nutrients, especially nitrogen (N) [1]
Fixation (BNF) by alfalfa and other legumes are generally associated with low soil inorganic N (SIN), high phosphorus availability, and adequate availability of micronutrients needed as co-factors of the nitrogenase enzyme that is responsible for catalyzing biological N fixation (BNF) [11]
Application of biochar, a soil amendment created by pyrolyzing biomass at a range of temperatures (350–1000 ◦ C), resulting in a carbon-rich, recalcitrant material that can persist in the soil for centuries [12,13], has been shown to reduce SIN [3], potentially forcing alfalfa to rely more on BNF for N requirements and leading to increased input of biologically-fixed N
Summary
A key facet of soil health and sustainable agriculture is supporting the ability of soils to retain and cycle nutrients, especially nitrogen (N) [1]. Diversifying cropping systems by adding a legume in rotation has been shown to enhance microbial biomass and bioavailable N, while reducing GHG emissions and external inputs [5,6], making legume addition a widely accepted practice for sustainable agriculture [7]. Application of biochar, a soil amendment created by pyrolyzing biomass at a range of temperatures (350–1000 ◦ C), resulting in a carbon-rich, recalcitrant material that can persist in the soil for centuries [12,13], has been shown to reduce SIN [3], potentially forcing alfalfa to rely more on BNF for N requirements and leading to increased input of biologically-fixed N. Biochar-induced increases in microbial activity [3] and in the bacterial to fungal ratio, which is more often found in high pH soils and with low biochar application rates [14], may lead to higher immobilization of inorganic N, due to greater N requirements by larger microbial communities, and by bacteria relative to fungi due to bacteria’s lower biomass C:N ratio
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