Measuring ecological restoration in conservation agricultural fields in the eastern Free State (South Africa) using nematodes and other soil bioindicators

Abstract

Conventional agriculture (Conv) has several adverse effects on soil health. For this reason, Conservation agriculture (CA) is implemented as an alternative to aid in restoring soil (ecosystem) health and functioning. However, its restorative impact remains largely unknown due to limited focus on the biological component of agricultural soils. Nematode-based indices (NBIs), namely the Maturity Index 2-5 (MI2-5), nematode faunal analysis, and nematode metabolic footprints (NMFs) were used to measure soil ecosystem health and functioning in three CA farmlands in Vrede and Reitz (eastern Free State, South Africa), before and during the summer growing season. Two reference sites at each study locality included a natural veld area (undisturbed positive reference) and a Conv farmland (disturbed negative reference). Other selected bioindicators included soil respiration (SR), active carbon (AC), and percentage total organic matter (% OM). A further knowledge gap regarding the impact of soil abiotic properties on soil ecosystem health and functioning necessitated the inclusion of soil texture, soil pH and EC, and soil macro and micronutrient analyses. In Vrede, the CA farmlands with a uniform soil texture and cash/cover crop rotation, displayed increased soil ecosystem health and improved soil food web structure as opposed to the Conv reference site (VConv). Farmlands receiving increased inorganic nitrogen (N) inputs presented larger Enrichment (EF) and Bacterivore footprints (BF). The reference site NVV presented the highest values for bioindicators, while the CA farmlands all presented a higher % OM than VConv but displayed varied values of SR and AC. Redundancy analyses (RDAs) indicated the negative correlation of inorganic N with the Structure index (SI) and MI2-5. In Reitz, the CA farmlands with varying soil textures were under cash crop rotation and displayed improved soil ecosystem health and functioning during the second sampling interval, while RConv presented increased soil ecosystem disturbance and decreased soil food web stability during the second sampling interval. A large Herbivore footprint (HF) was observed in most farmlands, especially during the growing season, and in particular for RConv. The impact of clay % on soil ecosystem functioning was confirmed in an RDA that highlighted positive correlations between clay % and bioindicators. Inorganic N was also positively correlated with the bioindicators. Fe also correlated with SI and MI 2-5 measurements prompting the need for further investigations into this trend. Results showed that the studied Vrede CA farmlands were undergoing a process of soil ecosystem restoration, while VConv displayed decreased soil ecosystem functioning. Farmers still utilizing Conv systems are therefore encouraged to consider the benefits of transitioning to CA to increase soil health and agricultural sustainability especially through inorganic N reductions. Despite the implementation of restorative agriculture practices like CA, the effect of soil texture differences on soil ecosystem health and functioning was evident in Reitz and must be considered when measuring soil ecosystem restoration.