Evidence for functional state transitions in intensively-managed soil ecosystems

Abstract

Soils are fundamental to terrestrial ecosystem functioning and food security, thus their resilience to disturbances is critical. Furthermore, they provide effective models of complex natural systems to explore resilience concepts over experimentally-tractable short timescales. We studied soils derived from experimental plots with different land-use histories of long-term grass, arable and fallow to determine whether regimes of extreme drying and re-wetting would tip the systems into alternative stable states, contingent on their historical management. Prior to disturbance, grass and arable soils produced similar respiration responses when processing an introduced complex carbon substrate. A distinct respiration response from fallow soil here indicated a different prior functional state. Initial dry:wet disturbances reduced the respiration in all soils, suggesting that the microbial community was perturbed such that its function was impaired. After 12 drying and rewetting cycles, despite the extreme disturbance regime, soil from the grass plots, and those that had recently been grass, adapted and returned to their prior functional state. Arable soils were less resilient and shifted towards a functional state more similar to that of the fallow soil. Hence repeated stresses can apparently induce persistent shifts in functional states in soils, which are influenced by management history.

Figure 1

The concept of ecological resilience illustrated by considering a ball in a basin of attraction in (a) a simple model system and (b) a complex system with multiple stable states. Whilst the system exists in one particular state at a given time (the black ball) this state is dynamic and will change depending on the variability of the surrounding environment (the arrows and grey balls indicate how the state may change). Due to self-organising mechanisms the system state tends to stay within a range (the cup) unless a disturbance is sufficient such that a resilience threshold is exceeded, when the system instead tends towards an alternative stable state.

Figure 2

Hierarchical clustering of the set of model parameters identified by fitting the model to soil respiration in each replicate after each cycle. Three distinct types of response were identified; Type 1 with well defined initial decay and secondary and tertiary respiration pulses, Type 2 typically with smaller initial decay and a later tertiary pulse and Type 3 characterised by small secondary and tertiary pulses. On the left the fitted models for all of the responses of each type are shown.

Figure 3

The types of respiration responses observed in response to substrate addition in different soil treatments, after an incubation period (cycle 0) and increasing cycles of drying and rewetting. The response type numbers and colour coding correspond to those identified by the hierarchical cluster analysis shown in Fig. 2. Cycle 0 shows the type of response prior to disturbance.

Figure 4

Illustration of the descriptive model of respiration that quantifies different features of the characteristic response via a series of parameter values.