Changes and influencing factors of topsoil organic carbon pool in the typical region of Haihe Plain：a case study of Jinnan District，Tianjin

Abstract

为探究海河平原典型区表层土壤有机碳库的变化及其主要影响因素，本研究基于 1980年和 2016年两期土壤调查数据，采用地统计学方法分析了天津市津南区土壤有机碳密度（SOCD）及土壤有机碳库的变化，运用线性回归和随机森林模型分析了土壤有机碳（SOC）主要影响因素。结果显示：津南区表层土壤有机碳库2016年储量为1.14×10⁶ t，比1980年增加了6.61×10⁴ t；SOCD与成土母质关系密切，不同时期SOCD都表现出冲积物>冲积-海积物>湖沼积物这一规律；两期SOCD高值区分布基本一致，主要分布在津南区西北部及中部咸水沽镇附近，1980—2016年SOCD变化有“北减南增”的趋势；有机碳变化（△SOC）受土壤理化性质及土地利用变更（LUC）共同影响，其中土壤全氮含量变化（△TN）、碳氮比（C/N）变化、氮磷比（N/P）变化与△SOC呈极显著正相关（P<0.01），土壤有机碳初始值（ISOC）、CaO含量、pH变化（△pH）与△SOC呈显著负相关（P<0.05）。研究表明，土壤有机碳库在农用地向城镇用地转换时表现为“碳源”，而农用地性质不变时表现为“碳汇”，△TN、C/N变化、N/P变化和LUC在SOC动态变化中起重要作用。

The changes in topsoil organic carbon pool and its main influencing factors were investigated in the typical area of the Haihe Plain, based on soil survey data obtained in 1980 and 2016. Geostatistical methods were employed to analyze the changes in soil organic carbon density（SOCD）and soil organic carbon pool. Linear regression and random forest models were used to analyze the primary influencing factors of soil organic carbon（SOC）. The results showed that the storage capacity of the topsoil organic carbon pool of 2016 in Jinnan District was 1.14×10⁶ t, which increased by 6.61×10⁴ t compared with that in 1980. SOCD was closely related to the soil-forming parent material, and the SOCD values were ranked as follows：alluvium>alluvial-marine deposit>lake marsh deposit in different periods. The distribution of high SOCD values in two periods was similar, mainly distributed in the northwest of Jinnan District and near Xianshuigu Town in the center of Jinnan District, and the variation of SOCD from 1980 to 2016 showed a decreasing and increasing trend in the north and south, respectively. The change in organic carbon（△SOC）was affected by soil physicochemical properties and land use change（LUC）, in which the changes in soil total nitrogen（△TN）, carbon to nitrogen ratio（C/N）, and nitrogen to phosphorus ratio（N/P）were significantly positively correlated with △SOC（P<0.01）, and initial soil organic carbon（ISOC）, CaO content, and change in pH（△pH）were significantly negatively correlated with △SOC（P<0.05）. The findings indicated that the SOC pool acts as“carbon source”when agricultural land is converted to urban land, whereas it serves as a“carbon sink”when the nature of the agricultural land remains unchanged, and the changes in △TN, C/N, N/P, and LUC play an important role in SOC dynamic changes.