亚热带红壤真菌生物量下降对于硝态氮微生物同化的影响

Compared with woodland soils, the major inorganic N in the agricultural soils is nitrate, which makes the risk of N losses from soil sharply increased after woodland was converted to agricultural use in humid subtropical zone. Nitrate microbial assimilation in soils could enhance the capacity of soil N retention and thus reduce the potential risk of N losses. Agricultural land use is thought to depress nitrate microbial assimilation probably by reducing fungal biomass in soil. In order to adopt homologues management measures, it is necessary to distinguish between nitrate immobilized by fungal and bacteria. However, the only method to assess the relative proportion of fungal and bacterial activities is the use of selective inhibitors, which is regarded as unreliable because the selective inhibitor may have non-target effects or other unintended consequences.. As important constituents in microbial cell walls, amino sugars are considered as a storage pool for the immobilized N. Additionally, amino sugars are reliable microbial residue biomarkers due to their different origins. Among the identified amino sugars, muramic acid (MurN) originates exclusively from bacteria. Glucosamine (GlcN) in soil is mainly in the form of chitin in fungal cell walls. The origin of soil galactosamine (GalN) is currently tricky. It was generally considered to be derived mainly from bacteria. Recently, an isotope labeling based gas chromatograph/mass spectrometry (GC/MS) was developed and it can be used to evaluate the isotope enrichment in amino sugars and to differentiate the newly synthesized microbial residues from the native portions in soil samples. This method offers an opportunity to distinguish between nitrate immobilized by fungal and bacteria in soil.. The objectives of this study were to investigate the effects of fungal biomass decrease on nitrate microbial assimilation in humid subtropical soils of China. Our hypothesis is that fungal would be reduced by intensive agricultural land use, which leads directly to the depression of nitrate microbial assimilation as compared with forest soils. In order to test the above hypothesis, 12 natural woodland and 12 agricultural soils were collected from typical subtropical zones in Heshan, Guangdong province to investigate the effects of land use on nitrate microbial assimilation and microbial community composition. Laboratory incubations of soil samples were conducted by using K15NO3 as N source. The pattern of isotope incorporation into each amino sugar was determined by GC/MS, and thus specific response of soil bacteria and fungal to nitrate were differentiated to distinguish between nitrate immobilized by fungal and bacteria in soil. The soil microbial community composition (fungal: bacterial dominance) was analyzed by phospholipid fatty acid (PLFAs) method.

土壤硝态氮微生物同化能力下降是导致亚热带地区农业利用红壤硝酸盐累积，氮素损失风险提高的重要原因。而农业利用红壤硝态氮微生物同化能力下降的原因之一可能是土壤真菌生物量减少。因此，区分真菌和细菌对硝态氮的同化能力，对于认清农业利用导致红壤硝态氮微生物同化能力下降的原因，采取相应治理措施至关重要。以往的相关研究因方法限制，无法有效区分真菌、细菌对无机氮同化的相对贡献。本研究拟采用近年来发展的基于稳定同位素示踪结合氨基糖异源性评价真菌、细菌对于底物利用相对贡献的方法，以广东鹤山为研究区，通过添加K15NO3室内培养，利用气相色谱-质谱联用技术测定不同土地利用方式红壤氨基糖含量及15N富集比例，区分真菌和细菌对于硝态氮同化的相对贡献。另外，结合土壤微生物群落结构分析，明确农业利用红壤真菌生物量下降是否直接导致硝态氮微生物同化能力下降，为合理利用土地，恢复土壤硝态氮同化能力，减少氮素损失提供科学依据。

土壤硝态氮微生物同化能力下降是导致亚热带地区农业利用红壤硝酸盐累积，氮素损失风险提高的重要原因。然而，农业利用对于真菌和细菌各自对于硝态氮同化能力的影响还不清楚。因此，区分真菌和细菌对硝态氮的同化能力，对于进一步认清农业利用导致红壤硝态氮微生物同化能力下降的原因，进而制定治理措施至关重要。因方法限制，以往研究无法有效区分真菌、细菌对无机氮的同化能力。本研究采用近年来发展的基于稳定同位素氨基糖探针技术(Amino sugar-SIP)来评价真菌、细菌对于底物利用相对贡献的方法，以广东鹤山为研究区，通过添加K15NO3室内培养，利用气相色谱-质谱联用技术测定不同土地利用方式土壤氨基糖含量及其15N富集比例，成功区分真菌和细菌对于硝态氮的同化能力。同时采用PLFA法分析了土壤真细菌生物量对于农业利用的响应。研究结果：农业利用使得亚热带森林红壤真菌和细菌对于硝态氮的同化能力同时显著下降，并且农业利用对于真菌NO3-同化活性的抑制程度高于细菌；细菌生物量下降可以解释细菌同化能力下降的41.2%，而真菌生物量仅能解释不足2%的真菌同化能力变异；农业利用所导致的土壤有机碳及碳氮比下降，有效磷，全磷以及pH升高可能是导致硝态氮同化能力下降的主要控制因子。研究意义：本研究不仅为合理利用土地，恢复农田土壤硝态氮同化能力，减少氮素损失提供科学依据，而且打破了常规采用选择性抑制剂的框架，创新性采用Amino sugar-SIP技术，为揭示环境变化如何通过影响土壤理化性质以及微生物群落结构，最终影响土壤真菌和细菌各自对于不同形态氮素同化能力开辟了一条新的有效解决途径。