Institute of Geography & Earth Sciences, Aberystwyth University, Aberystwyth, UK.
Philos Trans R Soc Lond B Biol Sci. 2012 Nov 19;367(1606):3076-86. doi: 10.1098/rstb.2012.0102.
Biological soil crusts (BSCs) are an important source of organic carbon, and affect a range of ecosystem functions in arid and semiarid environments. Yet the impact of grazing disturbance on crust properties and soil CO(2) efflux remain poorly studied, particularly in African ecosystems. The effects of burial under wind-blown sand, disaggregation and removal of BSCs on seasonal variations in soil CO(2) efflux, soil organic carbon, chlorophyll a and scytonemin were investigated at two sites in the Kalahari of southern Botswana. Field experiments were employed to isolate CO(2) efflux originating from BSCs in order to estimate the C exchange within the crust. Organic carbon was not evenly distributed through the soil profile but concentrated in the BSC. Soil CO(2) efflux was higher in Kalahari Sand than in calcrete soils, but rates varied significantly with seasonal changes in moisture and temperature. BSCs at both sites were a small net sink of C to the soil. Soil CO(2) efflux was significantly higher in sand soils where the BSC was removed, and on calcrete where the BSC was buried under sand. The BSC removal and burial under sand also significantly reduced chlorophyll a, organic carbon and scytonemin. Disaggregation of the soil crust, however, led to increases in chlorophyll a and organic carbon. The data confirm the importance of BSCs for C cycling in drylands and indicate intensive grazing, which destroys BSCs through trampling and burial, will adversely affect C sequestration and storage. Managed grazing, where soil surfaces are only lightly disturbed, would help maintain a positive carbon balance in African drylands.
生物土壤结皮（BSCs）是有机碳的重要来源，并且影响干旱和半干旱环境中的一系列生态系统功能。然而，放牧干扰对结皮层特性和土壤 CO2 排放的影响仍研究甚少，特别是在非洲生态系统中。在博茨瓦纳南部卡拉哈里的两个地点，通过风积沙掩埋、崩解和去除 BSCs 来研究它们对土壤 CO2 排放、土壤有机碳、叶绿素 a 和藻蓝蛋白季节性变化的影响。现场实验用于分离源自 BSCs 的 CO2 排放，以便估算结皮层内的 C 交换。有机碳在土壤剖面中分布不均匀，而是集中在 BSC 中。卡拉哈里沙中的土壤 CO2 排放高于方解石土壤，但速率随水分和温度的季节性变化而显著变化。两个地点的 BSCs 都是土壤的小型净碳汇。去除 BSCs 的沙质土壤中的土壤 CO2 排放明显较高，而在 BSCs 被沙掩埋的方解石土壤中也是如此。沙质土壤中 BSCs 的去除和掩埋以及方解石土壤中 BSCs 的掩埋也显著降低了叶绿素 a、有机碳和藻蓝蛋白。然而，土壤结皮的崩解会导致叶绿素 a 和有机碳的增加。这些数据证实了 BSCs 在干旱地区碳循环中的重要性，并表明密集放牧会通过践踏和掩埋破坏 BSCs，从而对 C 固存和存储产生不利影响。适度放牧，即土壤表面只受到轻微干扰，将有助于维持非洲干旱地区的碳平衡。
