模拟氮沉降对克氏针茅草原土壤有机碳的短期影响

为更好地了解天然草地土壤有机碳对氮沉降增加的响应,2011年在内蒙古太仆寺旗的克氏针茅(Stipa krylovii)草原上开展了模拟氮沉降的控制实验,设置对照(CK)和5个模拟氮沉降(NO-3)处理,分别为2(N1)、5(N2)、10(N3)、25(N4)和50 g N m-2a-1(N5)。生长季末,采集每个样地中0—2 cm和2—10 cm深度土壤进行有机碳含量及组成的分析,并进行实验室矿化培养。结果表明,土壤颗粒态有机碳(POC)对氮添加响应敏感,N1和N2处理下的POC含量高于CK,N3、N4和N5处理则低于CK。5个模拟氮沉降处理下的矿质结合态有机碳(MOC)含量均高于对照,但差别不显著。不同氮沉降水平下0—2 cm土层的碳矿化潜势为N2N1N4N3CKN5,且N1,N2,N3和N4处理均显著高于CK和N5;2—10 cm土层的碳矿化潜势为N2N1N3CKN4N5,N1、N2和N3显著高于CK、N4及N5。不同施氮处理对群落净第一性生产力有明显影响,N5的净第一性生产力和地上生物量显著低于对照和其它施氮处理,N1的0—10 cm地下生物量显著高于对照和其它处理,N5的凋落物量显著高于对照。模拟氮沉降短期内对土壤总有机碳(SOC)含量无显著影响。

Short-term effects of simulating nitrogen deposition on soil organic carbon in a Stipa krylovii steppe

In order to examine the short-term effects of simulated nitrogen (N) deposition on soil organic carbon (SOC) in Stipa krylovii steppe, an N addition experiment was set up in May 2011 on a typical steppe, Stipa krylovii community, in Inner Mongolia, China. The experiment included five simulated N (NO₃^-) deposition level treatments, namely N1 (2 g N m^-2 a^-1), N2 (5 g N m^-2 a^-1), N3 (10 g N m^-2 a^-1), N4 (25 g N m^-2 a^-1), and N5 (50 g N m^-2 a^-1), and a control (CK) treatment with no N addition, each with three replicates. At the end of the growing season in 2011, soils at 0-2 cm and 2-10 cm depth were sampled separately. The fractionation of SOC was determined, and the potential SOC mineralization was measured in an incubation experiment. The SOC fractionation results indicated that particulate organic carbon (POC) was more sensitive to N deposition than the mineral associated organic carbon (MOC). The POC in 0-2 cm and 2-10 cm soils was higher in the N1 and N2 treatments, but lower in the N3, N4, and N5 treatments than in the CK treatment. The POC was significantly higher in the N1 than the N4 and N5 treatments. The ratio of POC to SOC can reflect the stability of the SOC pool. In the 0-2 cm soil, the POC/SOC was significantly higher in the CK than the N4 and N5 treatments, and in the 2-10 cm soil it was significantly higher in the N5 treatment. During the 70 day incubation experiment, the potential SOC mineralization decreased rapidly with soil depth. In the 0-2 cm soils, the potential SOC mineralization decreased in the order: N2 > N1 > N4 > N3 > CK > N5, with significant differences between the N1, N2, N3 and N4 treatments on one hand and between the CK and N5 treatments on the other. In the 2-10 cm soils, the potential SOC mineralization decreased in the order: N2 > N1 > N3 > CK > N4 > N5, with significant differences between the N1, N2, and N3 treatments on one hand and between the CK, N4 and N5 treatments on the other. Thus, simulated N deposition caused an increase in SOC, in particular POC, at low N addition levels, whereas at high N input, POC, MOC and the accumulated SOC mineralization decreased. Net primary productivity (NPP) was also affected by N addition. The NPP and aboveground biomass level was significantly lower in the N5 treatment than in the CK and other treatments, the underground biomass at 0-10 cm was significantly higher in the N1 treatment than in the CK and other treatments, and the amount of litter was significantly higher in the N5 than in the CK treatment. The effects of N deposition on NPP, biomass and litter were the main factors that influenced the SOC fractionation and the potential SOC mineralization in the short term. However, results showed that after one growing season, there were no significant effects of simulated N deposition on the SOC of Stipa krylovii steppe.