Maize straw is more effective than maize straw biochar at improving soil N availability and gross N transformation rate

Abstract

Soil nitrogen (N) transformation is vital in determining farmland N availability. Although many studies have investigated the effect of biochar on N retention and loss via leaching and gaseous emission, few have determined the dynamics of gross N transformation during crop growth in long‐term biochar‐amended soils and compared the effect of the biochar with that of its feedstock. In this study, we conducted a five‐time field measurement of soil gross N turnover rates via 15N isotope pool dilution during maize growth in 2021. Three treatments were employed, including no amendment (CK), biochar (BC) and straw (ST) applied annually at rates of 2.63 and 7.50 t ha−1, respectively, since 2013. The results showed that biochar did not change the rate of gross N mineralization when compared with CK, but straw increased it by 139% in August, resulting in significantly higher cumulative gross N mineralization than BC and CK (701 vs 489 and 499 mg kg−1 in 200 d). The inconsistent influence was attributed to the fact that inherent biochar‐N was recalcitrant that cannot be mineralized like the straw. The gross nitrification rate was decreased by 72.9% and 77.4% by biochar and straw application in June relative to CK, but then it was increased from July to August in ST as a result of the elevated gross N mineralization rate. The decreased nitrification in BC was an outcome of the synergetic effect of low ammonium pool (−59.4%) and high gross ammonium immobilization rate (+263%), which was likely due to excessive fertilizer N loss and abiotic adsorption to biochar. Meanwhile, biochar amendment inhibited bacterial 16S and fungal ITS genes, as well as ureC and bacterial and archaea‐amoA gene copies. In conclusion, straw is a more effective than biochar at improving soil N transformation and availability in the long term.This article is protected by copyright. All rights reserved.