Synthetic soil crusts against green-desert transitions: a spatial model

Semiarid ecosystems are threatened by global warming due to longer dehydration times and increasing soil degradation. Mounting evidences indicate that, given the current trends, drylands are likely to expand and possibly experience catastrophic shifts from vegetated to desert states. Here we explore a recent suggestion based on the concept of ecosystem terraformation, where a synthetic organism is used to counterbalance some of the nonlinear effects causing the presence of such tipping points. Using an explicit spatial model incorporating facilitation and considering a simplification of states found in semiarid ecosystems i.e., vegetation, fertile and desert soil, we investigate how engineered microorganisms can shape the fate of these ecosystems. Specifically, two different, but complementary, terraformation strategies are proposed: Cooperation-based: C-terraformation; and Dispersion-based: D-terraformation. The first strategy involves the use of soil synthetic microorganisms to introduce cooperative loops (facilitation) with the vegetation. The second one involves the introduction of engineered microorganisms improving their dispersal capacity, thus facilitating the transition from desert to fertile soil. We show that small modifications enhancing cooperative loops can effectively change the location of the critical transition found at increasing soil degradation rates, also identifying a stronger protection against soil degradation by using the D-terraformation strategy. The same results are found in a mean field model providing insights into the transitions and dynamics tied to these terraformation strategies. The potential consequences and extensions of these models are discussed.

半干旱生态系统正面临全球变暖的威胁，具体表现为脱水周期延长与土壤退化加剧。越来越多的研究证据表明，若维持当前发展趋势，旱地面积或将持续扩张，甚至可能经历从植被覆盖态到荒漠态的灾难性状态转换。本文基于生态系统改造（ecosystem terraformation）概念，对一项新近提出的解决方案展开探讨：借助合成生物抵消引发此类临界点出现的部分非线性效应。本研究采用融入物种促进作用的显式空间模型，同时简化半干旱生态系统的三类典型状态——植被覆盖、肥沃土壤与荒漠土壤，以此探究工程化微生物如何调控此类生态系统的存续走向。具体而言，本文提出两种互补的改造策略：基于协同作用的C-改造（C-terraformation），以及基于扩散能力的D-改造（D-terraformation）。第一种策略为利用土壤合成微生物，构建与植被之间的协同循环（即物种促进作用）；第二种策略则通过引入工程化微生物以提升其扩散能力，进而推动荒漠土壤向肥沃土壤的状态转换。研究结果表明，仅需对协同循环进行小幅增强，即可有效调整土壤退化速率提升时触发临界转换的阈值；同时还发现，采用D-改造策略可对土壤退化提供更强的抵御能力。通过平均场模型亦可得到一致结果，该模型可帮助我们深入理解与上述改造策略相关的状态转换与动态变化过程。本文最后讨论了上述模型的潜在应用价值与拓展方向。