开垦对绿洲农田碳氮累积及其与作物产量关系的影响

以新疆策勒绿洲近百年来不同开垦年限农田为研究对象,采用空间序列换算时间序列的方法,研究绿洲农田开垦过程中土壤有机碳和全氮密度、碳氮比(C/N)及速效氮含量的垂直变化特征,并探讨了农田土壤碳氮变化与作物产量的关系。结果表明:荒漠土壤开垦后,显著增加了表层土壤(0-20 cm)有机碳和全氮密度,随开垦年限延长对深层土壤(40-200 cm)有机碳密度也有一定的影响,如在开垦30 a左右时下降了36.4%,但在100 a左右时则增加了52.0%。耕层土壤C/N随开垦年限延长而明显增加,深层土壤除100 a农田外其它均有不同程度下降;不同土层C/N与速效氮含量呈负相关关系,仅在开垦初期(0-10 a)达到显著水平。不同年限农田的玉米产量存在显著差异,且和有机碳及全氮密度(0-200 cm)均呈显著正相关;棉花除100和10 a农田产量差异较小外,在其它农田间均达显著水平,但和有机碳及全氮密度无明显相关性。由此可见,在现有投入条件下,提高土壤碳氮累积量对增加玉米产量仍有十分重要作用,但对棉花产量的影响不明显。

Effect of cultivation on soil organic carbon and total nitrogen accumulation in Cele oasis croplands and their relation to crop yield

Soil organic carbon storage and total nitrogen contents are not only important indicators of soil quality and sustainable crop production, but are also an option for offsetting increasing atmospheric CO₂ and N₂O concentrations. Cultivation often causes deterioration of physical soil conditions and reduces nutrient status and humus content, and therefore is considered the main cause of changes in soil organic carbon and nitrogen. Most studies show a decline in soil carbon after cultivation, averaging about 30%. However, some research has suggested that organic carbon contents significantly increase after soils with low natural organic matter levels are converted to cropland. Therefore, soil organic carbon storage and the dynamics of carbon change in cropland have become important issues in evaluating the impact of agricultural management. However, many researchers pay more attention to changes in soil carbon stocks in the plough layer than to changes in deep soil layers. Twenty cropland sites in the Cele oasis, which have been cultivated for up to 100 years, were selected to study the effects of cultivation on changes in the vertical distribution of soil organic carbon, total nitrogen, and available nitrogen by using the method of trading space with time. Based on differences in soil organic carbon and total nitrogen accumulation, five sites representing 100, 80, 30, 15 and 10 years of cultivation were chosen to investigate relationships between crop yield and soil organic carbon or total nitrogen. Soil organic carbon and total nitrogen density in the surface soil layers increased significantly with longtime cultivation. Soil organic carbon densities (0-20 cm) in croplands cultivated for 100, 80, 30, 15 and 10 years were, respectively, 231.7%, 302.9%, 146.3%, 116.6%, and 130.5% higher than those in an uncultivated desert soil. Corresponding values for total nitrogen density were, respectively, 160.1%, 217.6%, 123.6%, 106.5%, and 125.1%. The organic carbon density in deep soil layers (40-200 cm) was also influenced by longtime cultivation, being 36.4% lower after 30 years' cultivation than that in the desert soil. However, in the 100-year cropland it increased by 52.0%. Similar results were not found for total nitrogen density. The C/N ratio in the 0-40 cm soil layers of the sites cultivated for 100, 80, 30, 15, and 10 years increased by 28.3%, 23.0%, 15.7%, 10.4%, and 6.5%, respectively, compared with that in the desert soil. However, the C/N ratio decreased in deep soil layers of the sites cultivated for 0 to 80 years. Significant negative correlations between C/N ratios and soil available nitrogen in the different soil layers were present only in the desert and 10-year cropland soils. There were significant differences in maize yield in the different croplands. In addition, maize yield was significantly positively correlated with soil organic carbon and total nitrogen density in the 0-200 cm layers, but a corresponding correlation was not found for cotton yield. This suggests that increases in soil organic carbon and total nitrogen were very important for improving maize yield at the Cele oasis, but this was not the case for cotton yield.