喀斯特石漠化地区不同退耕年限下桂牧1号杂交象草植物-土壤-微生物生态化学计量特征

耕地农作物种植与退耕地种草养畜是喀斯特主要的农业生产模式。以喀斯特地区农耕玉米地为对照,研究退耕还草1、5、7a(恢复初期、旺盛期、衰退期)3种年限下桂牧1号杂交象草地植物-土壤-微生物C、N、P生态化学计量特征及内在关联。结果表明:1)牧草地植物地上部分N、P含量均为5a牧草1a牧草7a牧草,C含量则刚好相反;3种退耕年限牧草地植物地上部分C∶N、C∶P、N∶P分别为26.50—33.91、631.70—2254.33、23.89—67.21,且均表现为7a牧草1a牧草5a牧草。2)土壤表层(0—10 cm)C、N、P含量均以玉米地最低,3种退耕年限牧草地中则均为5a牧草地最低;土壤C∶N、C∶P、N∶P在玉米及退耕牧草地之间均无显著差异(P0.05),平均值分别为9.20,27.88,3.38。3)玉米及牧草地土壤MB_C、MB_N、MB_P含量存在显著差异(P0.05);玉米地MB_C/SOC、MB_N/TN、MB_P/TP均高于牧草地,3种退耕年限牧草地中,则均为5a牧草地最高。4)MB_C、MB_P与土壤C、P含量分别呈显著线性正相关(P0.05);植物C、C∶N与土壤C、N含量均呈极显著线性负相关(P0.01)。分析表明,退耕还草地中植物与土壤系统C-N-P化学计量比表现出不一致的时间变化特征,且牧草地植物受P限制严重,尤以恢复旺盛期为甚。

Ecological stoichiometric characteristics of plants, soil, and microbes of Pennisetum purpureum cv. Guimu-1 pastures at different rehabilitation ages in a karst rocky desertification region

Ecological stoichiometry has become the focus of research in ecological sciences in recent years and many studies have examined the carbon:nitrogen:phosphorus (C:N:P) stoichiometry of plants, soil, or litter in forest communities. However, few studies have explored the stoichiometric characteristics of herbs at different ages. Elephant grass, Pennisetum purpureum cv. Guimu-1, is a tall perennial C4 grass that can withstand repeated cutting and regenerates rapidly, and has been widely cultivated in northwest Guangxi, China because of the "grain for green" policy. For enhanced understanding of the nutrient cycling characteristics of artificial forage at different time scales in fragile karst ecosystems, we investigated the C:N:P stoichiometric characteristics of plant-soil-microbe interactions in Guimu-1 pastures of different growth ages. Corn and three different restoration stage pastures (1-, 5-, and 7-y-old Guimu-1 pastures, representing initial restoration, vigorous restoration, and degenerating stages, respectively, after returning farmland to grassland) were chosen in northwest Guangxi, China to determine the C, N, and P contents and the C:N:P stoichiometry. The results showed that:(1) the C, N, P contents and C:N:P ratios in the aerial portion of plants varied with different artificial Guimu-1 pasture ages. The N and P contents of the plant aerial portion in different years all followed the same order:5-y-old < 1-y-old < 7-y-old pastures, whereas the C content followed the reverse order. Plant C:N, C:P, and N:P ratios, which varied over a large range from 26.50 to 33.91, 631.70 to 2254.33, and 23.89 to 67.21, respectively, were all in the order of 7-y-old < 1-y-old < 5-y-old pastures. (2) Soil C, N, and P in the topsoil layer (0-10 cm) all showed the same order:corn field < 5-y-old < 1-y-old < 7-y-old pastures. However, there were no significant differences among the soil C:N, C:P, and N:P ratios for the four herb types (P > 0.05), with average values of 9.20, 27.88, 3.38, respectively. (3) Soil microbial C biomass (MB_C), microbial N biomass (MB_N), and microbial P biomass (MB_P) showed significant differences among the different plant types (P < 0.05). The corn field had the highest MB_C/soil organic C, MB_N/total N, and MB_P/total P ratios among the four plant types, while the 5-y-old pasture had the highest ratios among the three different aged pastures. (4) Correlation analysis showed that:(a) there were significant positive linear correlations between MB_C and soil C, and MB_P and soil P (P < 0.05); (b) there were highly significant negative linear correlations between plant and soil C, plant C and soil N, plant C:N and soil C, and plant C:N and soil N (P < 0.01); and (c) there were highly significant positive linear correlations between plant N and soil C, and plant and soil N (P < 0.01). Thus, the determination of C, N, and P stoichiometric characteristics of plant-soil-microbe interactions in the present study showed that in karst ecosystems, croplands were more limited by N than by P, whereas artificial pastures in rehabilitated land were mainly limited by P, especially during the vigorous restoration stage. Plant C:N:P stoichiometric characteristics in pastures at different ages showed a higher temporal variability than those of soil.