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Heterotrophic nitrification is regarded as an eternal mystery in the nitrogen (N) cycle, although it was first reported more than 100 years ago. In this review, we discuss microbial mechanisms driving heterotrophic nitrification of organic N (OHORG) in soil and their modulations by pH, and carbon (C) and N contents. In acidic and oligotrophic soils, OHORG may occur as endogenous respiration or during oxidation of recalcitrant organics and cell lysis. Likely in soils with a low C:N ratio and a high pH, C limitation of N immobilization creates conditions for the transformation of dissolved organic N to nitrate (NO3−). Fungi with a deficiency in ammonia mono-oxygenase can drive OHORG. Heterotrophic nitrifiers include not only acid-tolerant, nitrophobic species, but also acid-sensitive, nitrotolerant species. There is concern about OHORG in soil because of its contribution to NO3− production and nitrous oxide (N2O) emissions. Therefore, reliable measurements of soil heterotrophic nitrifying activity are needed. We propose more biochemical surveys to understand the pathway for OHORG in ecologically relevant species, considering the challenges we are facing in managing heterotrophically derived nitrification to prevent N losses from cropland, forest, and grassland soils. [ABSTRACT FROM AUTHOR]

Biochar amendment has complex impacts on greenhouse gas emissions, crop production and economic benefit. However, few studies have comprehensively investigated the effects of biochar amendment in coastal saline rice fields. Thus, a biochar amendment field experiment was established in a coastal saline rice field in China to estimate the CH4 and N2O emissions, global warming potential (GWP), greenhouse gas intensity (GHGI), and net ecosystem economic budget (NEEB) of the biochar amendment during the rice growing season in 2017. There were six treatments (N0B0, N0B1, N0B2, N1B0, N1B1, N1B2) with different N fertilizer levels of 0 and 300 kg N ha−1 and biochar rates of 0, 20, and 40 t ha−1. The results showed that the application of N fertilizer increased N2O emissions and rice yield by 128.3% (p < 0.001) and 44.4% (p < 0.001), respectively, while decreased the GHGI by 20.5% (p < 0.01); additionally, there were no significant effects on the CH4 emissions and GWP compared with the treatments without N fertilizer. Although biochar amendment significantly increased the N2O emissions and rice yield by 13.7–38.1% and 31.5–34.9%, respectively, biochar amendment had no significant effects on CH4 emissions, GWP, and GHGI relative to the treatments without biochar amendment. From an economic perspective, N fertilizer significantly increased the NEEB by 135.5%, relative to the treatments without N fertilizer. Due to the high price of biochar and the large quantity applied, biochar amendment significantly reduced the NEEB by 99.8–229.3% compared with the treatments without biochar amendment. Considering the different characters between field-aged biochar and fresh biochar. Thus, long-term observations are needed to evaluate the environmental and economic profits affected by biochar and N fertilizer. [ABSTRACT FROM AUTHOR]

Few studies have comprehensively evaluated the method of estimating the net ecosystem carbon budget (NECB). We compared two approaches for studying the NECB components on the crop seasonal scale as validated by the soil organic carbon (SOC) changes measured over the 5-year period of 2009-2014. The field trial was initiated with four integrated soil-crop system management (ISSM) practices at different nitrogen application rates relative to the local farmer's practices (FP) rate, namely, N1 (25 % reduction), N2 (10 % reduction), N3 (FP rate) and N4 (25 % increase) with no nitrogen (NN) and FP as the controls. Compared with the FP, the four ISSM scenarios of N1, N2, N3 and N4 significantly increased rice yields by 9.5, 19, 33 and 41 %, while increasing the agronomic nitrogen use efficiency (NUE) by 71, 75, 99 and 79 %, respectively. The SOC sequestration potentials were estimated to be −0.15 to 0.35 Mg C ha year from the net primary production minus heterotrophic respiration approach and −0.32 to 0.67 Mg C ha year from the gross primary production minus ecosystem respiration approach for the 2010-2011 rice-wheat annual cycle. Similarly, the annual topsoil carbon sequestration rate over 2009-2014 was measured to be −0.22 Mg C ha year for the NN plot and 0.13-0.42 Mg C ha year for the five fertilized treatments. Both NECB approaches provided a sound basis for accurate assessment of the SOC changes. Compared to the SOC sequestration rate from the FP, the proposed N3 and N4 scenarios increased the SOC sequestration rates while also improving rice yield and NUE. [ABSTRACT FROM AUTHOR]