Linkages of soil and microbial stoichiometry to crop nitrogen use efficiency: Evidence from a long-term nitrogen addition experiment

Abstract

Increasing crop nitrogen use efficiency (NUE) has important implications for food security and agricultural sustainability. Changes in nutrient availability due to stoichiometric imbalances in soil under long-term application of nitrogen (N) can limit crop NUE and yield. However, little is known about the linkages across stoichiometric balance, N fertilizer application, and N uptake. We investigated the changes in soil stoichiometry, microbial community, and crop NUE relative to N fertilizer application in a 16-year field experiment in which five levels (0, 70, 140, 210 and 280 kg N ha−1) of mineral N fertilizer treatment were applied to wheat and maize cropland. The results showed that the P storage decreased from 4.3 Mg P ha–1 under the fertilizer dose of 0 kg N ha−1 to 3.5 Mg P ha–1 under the fertilizer dose of 280 kg N ha−1. Thus, long-term N application increased soil C/P and N/P ratios, in a marked decrease in the content of soil available P. The microbial community based on phospholipid fatty acids (PLFAs) increased with increasing N addition rates, from 0 to 140 kg N ha−1, but significantly decreased at application rates above 210 kg N ha−1. Thus, applying N at a rate of 140 kg ha−1 resulted in the N threshold rate for microorganism survival being reached or exceeded. The crop NUE peak occurred at the urea application rate of 140 kg N ha−1 (60.4 %), after which NUE declined. The soil elementary and enzymatic stoichiometric ratios under long-term N addition directly affected crop NUE in the topsoil. Soil elementary stoichiometric ratios affected crop NUE by indirectly altering microbial biomass of the subsoil. Overall, soil stoichiometric imbalances under long-term N addition were the key factors driving NUE across N gradients.