Effect of biochar addition on soil active nitrogen fractions

We carried out a field experiment on BC addition in abandoned grasslands on the Loess Plateau to elucidate the effects of BC application on soil active N fractions (e.g., DON, MBN, NH4+-N, and NO3--N) and soil properties (e.g., physicochemical properties, microbial biomass, and related enzyme activities, etc.) in the abandoned grasslands on the Loess Plateau. We hypothesized that (1) BC addition could significantly facilitate the accumulation of soil active N fractions, but excessive addition of BC might reduce this facilitation, and (2) BC addition led to significant variations in the soil factors that influence the characteristics of accumulation and transformation of soil AON and DIN fractions.
Soil water content (SWC) and temperature (ST) were determined in situ by the drying differential method and thermometer (JM624, Jinming Instrument Co., Tianjin, China), respectively (Xie et al., 2024). SOC and dissolved organic carbon (DOC) were determined through the K2Cr2O7 oxidation method, while TN and TP were determined by the Kjeldahl method and molybdenum antimony colorimetric method, respectively (Liu et al., 2022; Cao et al., 2023). In addition, NH4+-N, NO3--N, and dissolved total nitrogen (DTN) were determined by KCl solution leaching-Kjeldahl nitrogen determination and UV spectrophotometry, respectively (Zhang et al., 2022b; Chen et al., 2024a). DON was the difference between the DTN and the contents of NH4+-N and NO3--N. Microbial biomass carbon (MBC) and MBN were determined by chloroform fumigation-K2SO4 extraction, and microbial biomass phosphorus (MBP) was determined by chloroform fumigation-NaHCO3 extraction (Zhang et al., 2022b; Mao et al., 2024). Urease (Ure) was determined by the urease colorimetric method (Ren et al., 2016), and β-1,4-N-acetylglucosaminidase (NAG) was determined by the p-nitrophenol colorimetric method of Liu et al. (Liu et al., 2023).

本研究在黄土高原弃耕草地开展了生物炭（Biochar, BC）添加野外原位试验，旨在阐明生物炭添加对黄土高原弃耕草地土壤活性氮组分（如溶解态有机氮（Dissolved Organic Nitrogen, DON）、微生物量氮（Microbial Biomass Nitrogen, MBN）、铵态氮（NH₄⁺-N）与硝态氮（NO₃⁻-N））及土壤性质（如理化性质、微生物量及相关酶活性等）的影响。本研究提出如下假说：（1）生物炭添加可显著促进土壤活性氮组分的累积，但过量施加生物炭可能会削弱该促进作用；（2）生物炭添加会显著改变影响土壤活性有机氮（Active Organic Nitrogen, AON）与溶解态无机氮（Dissolved Inorganic Nitrogen, DIN）组分累积与转化的土壤因子。
土壤含水量（Soil Water Content, SWC）与土壤温度（Soil Temperature, ST）分别采用烘干差减法与JM624型温度计（天津津明仪器有限公司，中国天津）原位测定（谢等，2024）。土壤有机碳（Soil Organic Carbon, SOC）与溶解态有机碳（Dissolved Organic Carbon, DOC）采用重铬酸钾氧化法测定，全氮（Total Nitrogen, TN）与全磷（Total Phosphorus, TP）分别采用凯氏定氮法与钼锑抗比色法测定（刘等，2022；曹等，2023）。此外，铵态氮、硝态氮与溶解态总氮（Dissolved Total Nitrogen, DTN）分别采用氯化钾溶液浸提-凯氏定氮法与紫外分光光度法测定（张等，2022b；陈等，2024a）；溶解态有机氮为溶解态总氮与铵态氮、硝态氮含量的差值。微生物量碳（Microbial Biomass Carbon, MBC）与微生物量氮采用氯仿熏蒸-硫酸钾浸提法测定，微生物量磷（Microbial Biomass Phosphorus, MBP）采用氯仿熏蒸-碳酸氢钠浸提法测定（张等，2022b；毛等，2024）。脲酶（Urease, Ure）采用脲酶比色法测定（任等，2016），β-1,4-N-乙酰氨基葡萄糖苷酶（β-1,4-N-acetylglucosaminidase, NAG）采用刘等（2023）提出的对硝基苯酚比色法测定。