Department of Ecosystem Science and Management, Texas A&M University, College Station, TX, USA.
Glob Chang Biol. 2018 May;24(5):1992-2007. doi: 10.1111/gcb.14048. Epub 2018 Feb 7.
Soil carbon, nitrogen, and phosphorus cycles are strongly interlinked and controlled through biological processes, and the phosphorus cycle is further controlled through geochemical processes. In dryland ecosystems, woody encroachment often modifies soil carbon, nitrogen, and phosphorus stores, although it remains unknown if these three elements change proportionally in response to this vegetation change. We evaluated proportional changes and spatial patterns of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) concentrations following woody encroachment by taking spatially explicit soil cores to a depth of 1.2 m across a subtropical savanna landscape which has undergone encroachment by Prosopis glandulosa (an N fixer) and other woody species during the past century in southern Texas, USA. SOC and TN were coupled with respect to increasing magnitudes and spatial patterns throughout the soil profile following woody encroachment, while TP increased slower than SOC and TN in topmost surface soils (0-5 cm) but faster in subsurface soils (15-120 cm). Spatial patterns of TP strongly resembled those of vegetation cover throughout the soil profile, but differed from those of SOC and TN, especially in subsurface soils. The encroachment of woody species dominated by N -fixing trees into this P-limited ecosystem resulted in the accumulation of proportionally less soil P compared to C and N in surface soils; however, proportionally more P accrued in deeper portions of the soil profile beneath woody patches where alkaline soil pH and high carbonate concentrations would favor precipitation of P as relatively insoluble calcium phosphates. This imbalanced relationship highlights that the relative importance of biotic vs. abiotic mechanisms controlling C and N vs. P accumulation following vegetation change may vary with depth. Our findings suggest that efforts to incorporate effects of land cover changes into coupled climate-biogeochemical models should attempt to represent C-N-P imbalances that may arise following vegetation change.
土壤碳、氮和磷循环密切相关，受到生物过程的控制，而磷循环进一步受到地球化学过程的控制。在旱地生态系统中，木本植物的入侵常常会改变土壤碳、氮和磷的储量，尽管目前尚不清楚这三种元素是否会按照比例响应这种植被变化而发生变化。我们评估了在德克萨斯州南部，过去一个世纪以来，由刺槐（一种固氮植物）和其他木本物种入侵引起的亚热带草原景观中，土壤有机碳（SOC）、总氮（TN）和总磷（TP）浓度的比例变化和空间格局，通过在 1.2 米的深度内采集空间显式土壤芯，跨越整个土壤剖面。SOC 和 TN 随着木本植物的入侵而在整个土壤剖面中以增加的幅度和空间模式耦合，而 TP 在最上层土壤（0-5cm）中比 SOC 和 TN 增加得慢，但在亚表层土壤（15-120cm）中增加得快。TP 的空间格局与整个土壤剖面中的植被覆盖模式强烈相似，但与 SOC 和 TN 不同，特别是在亚表层土壤中。以固氮树种为主的木本物种入侵这个磷限制的生态系统，导致与 C 和 N 相比，表层土壤中积累的土壤 P 比例减少；然而，在木本斑块下的土壤剖面较深部分，由于碱性土壤 pH 值和高碳酸盐浓度有利于 P 作为相对不溶性的钙磷酸盐沉淀，因此积累的 P 比例增加。这种不平衡的关系突出表明，控制植被变化后 C、N 和 P 积累的生物和非生物机制的相对重要性可能随深度而变化。我们的研究结果表明，在将土地覆盖变化的影响纳入耦合气候-生物地球化学模型时，应尝试代表植被变化后可能出现的 C-N-P 失衡。
