桂西北喀斯特山区土壤有机碳组分对农田利用方式转变的响应

Soil organic carbon and its fractions are the key indexes to assess soil fertility and also the important indexes to judge land degradation and restoration. However, very limited information is available in terms of the effects of cropland use change on soil organic carbon and its fractions. In this project, some typical cropland types, including maize–soybean rotation fields and three types of conservation agriculture systems (i.e., sugarcane, mulberry, and forage grass), were selected. The three types of conservation agriculture systems were the replacements of maize-soybean field. Firstly, we measured the soil organic carbon pattern of different water -size aggregates and reveal the response of physical stability carbon fractions to cropland use change. Secondly, we determined the patterns of permanganate oxidizable carbon (POXC) and chemical structure components of soil aggregates, unravel the response of chemical stability carbon fractions to cropland use change. Lastly, we measured the microbial biomass carbon/nitrogen, microbial community structure and seven extracellular enzyme activities in different size aggregates to reveal the microbial patterns of aggregates in different cropland use types. In Addition, we also determined the belowground root biomass and other soil physicochemical properties to analysis the driving mechanism of different stability soil carbon fractions change in a karst region of northwest Guangxi. The results of the study will contribute to understanding the influence of cropland use change on soil carbon pool in northwest Guangxi, in China, and provide scientific and technical support for the selection of conservation agriculture systems and the prevention and control of rocky desertification in karst area in China.

土壤有机碳及其组分是评估土壤肥力的关键指标，也是衡量土地退化或恢复的重要指标。然而目前在西南喀斯特山区很少有研究评估土壤有机碳及其组分对农田利用方式转变的响应。项目以桂西北喀斯特山区玉米-大豆轮作地及由其转变而来的甘蔗地、桑园和牧草地（种植桂牧一号）为对象，首先分析不同利用方式下土壤不同粒径团聚体组分有机碳，揭示土壤不同粒径团聚体有机碳库及物理稳定性有机碳组分对农田利用方式转变的响应特征；在此基础上，通过对土壤不同粒径团聚体化学稳定性碳组分及化学结构组成分析，阐明土壤不同粒径团聚体化学稳定性有机碳组分对农田利用方式转变的响应特征；最后结合细根生物量、土壤不同粒径团聚体中微生物指标和胞外酶活活性分析阐释农田利用方式转变对土壤不同粒径团聚体有机碳影响的驱动机制。结果将有助于深入理解桂西北喀斯特山区农田利用方式转变对土壤有机碳库的影响，为喀斯特山区保护性耕作方式甄别及石漠化防治提供科技支撑。

土壤有机碳及其组分是评估土壤肥力的关键指标，也是衡量土地退化或恢复的重要指标。然而目前在西南喀斯特山区很少有研究评估土壤有机碳及其组分对农田利用方式转变的响应。项目以桂西北喀斯特山区玉米-大豆轮作地及由其转变而来的甘蔗地、桑园和牧草地为对象，测定不同农田利用方式下土壤全土及团聚体组分有机碳含量和其它指标，研究结果如下：.1、湿季农田和森林土壤有机碳、氮、磷含量和储量显著高于干季土壤，玉米-大豆轮作地转换成牧草种植后有利于提升 0-50 cm土壤有机碳、氮储量，而甘蔗和桑园地土壤有机碳、氮、磷含量及储量并未显著增加。总体上，次生林和牧草地土壤微生物量碳、氮含量、PLFAs 总丰度及各功能性微生物群落丰度均显著高于其它三种农田利用模式。土壤有机碳、氮、磷含量的表层积聚程度显著低于土壤微生物量碳、氮，土壤有机碳含量是影响四种农田模式下土壤养分表层积聚、微生物群落丰度和结构的重要影响因子。.2、四种农田模式下0-30cm土壤有机碳氮储量主要以大团聚体（>2 mm）（65-75%）为主，玉米-大豆轮作地转换为牧草种植显著增加了土壤各粒径团聚体有机碳、氮储量，尤其是大团聚体。土壤总有机碳、氮储量增加主要与大团聚体有机碳、氮储量增加有关，进一步取决于大团聚体碳、氮含量的增加而不是团聚体数量的增加。.3、土壤各粒径团聚体易氧化有机碳含量随粒径增大而增大。牧草种植显著增加了团聚体POXC含量，尤其是大团聚体（>2 mm）。土壤化学稳定碳含量是土壤总有机碳的主要组成部分，占比达到了88-94%。农田利用模式转换对全土及各粒径团聚体稳定性有机碳含量无显著影响。大体上，各粒径团聚体化学稳定性有机碳含量在牧草地最高。.4、次生林土壤细根生物量明显高于农田土壤。四种农田土壤中，牧草地细根生物量最高，玉米-大豆轮作地最低。牧草细根碳、氮和磷储量显著高于其它三种农田利用模式，有利于为土壤提供较多的碳氮输入，进而促进牧草地土壤碳、氮库水平的提升。