Abstract :
[en] Soil health is essential for sustaining multiple soil functions for plants, animals, and humans. Nowadays, soil health assessment focuses on multiple soil functions, including crop productivity, carbon storage, biodiversity maintenance, and water quality. There is a lack of an indicator system of soil biota as an effective tool in soil health assessment. The approaches to construct a minimum data set (MDS) and the quantification of soil health have merits and demerits. Developing a more intuitive soil health assessment method can provide more information and management recommendations for stakeholders.
In this study, based on a 29-year field experiment, we constructed a standard soil health indicator selection and soil health assessment method to characterize soil health. This method was also utilized on farm scales to investigate the soil health difference between large-scale farming and smallholder farming. To integrate soil biological indicators into the conventional indicator system, we sampled soils from 11 long-term fertilization experiments in different climate zones in China, including different fertilization strategies. The sensitivity and effectiveness of the indices based on the biological indicators were assessed. The main results of this thesis are as follows:
(1) Long-term application of manure maintained soil pH, significantly increased water-stable aggregates, total and available nutrients, microbial biomass and community (bacteria, fungi, and actinomycetes, etc.), and enzyme activities compared with treatments under chemical fertilizers. Edaphic factors of soil organic carbon (SOC), available phosphorous (AP), fungi, cation exchange capacity, and clay content were identified as key indicators of soil health evaluation by network analysis. The soil health indices (SHIs) based on the SHI-area method of chemical fertilizer treatments were 39-52% lower than that of natural vegetation recovery, with carbon and nutrient cycling, soil biodiversity maintenance, and productivity at low ecosystem function levels. Manure application enhanced soil health by 150-196% compared to unfertilized control by improving SOC, P availability, and sustaining ecosystem functions with carbon and nutrient cycling, soil biodiversity maintenance, buffering and filtering capacity, and productivity at relatively high ecosystem function levels. Soil functions showed sensitive or sluggish responses to the increase in soil health. Manuring improved soil functions and soil health simultaneously, indicating a synergistic relationship. These findings highlight the significance of manure application to improve soil health and sustain functions in intensive agricultural systems.
(2) Different management practices altered soil physical, chemical, and biological indicators. The aggregate stability, soil organic matter (OM), available nutrients, and microbial biomass under large-scale management were significantly higher. Large-scale farming with organic compost altered microbial community and structure with a higher ratio of Fungi to Bacteria (F/B) and a lower ratio of Gram-positive bacteria to Gram-negative bacteria (G+/G-). The soil fertility index and biological index were, respectively, 36.5% and 24.2% higher in large-scale farms than in conventional fields. However, there was no significant difference in soil environmental indices between the two management modes. Network analysis was used to select key indicators. Bulk density, penetration resistance, mean weight diameter, soil OM, DOC, earthworm biomass, gram-negative bacteria, and bacteria were identified by network analysis as key indicators of soil health assessment. The soil health index based on these indicators of large-scale farming was 23.4% higher than that of conventional management by smallholders, with higher structural stability, total and active organic carbon, and quantities of soil fauna and microorganisms. Similarly, the farmland quality index and soil health index based on all indicators under large-scale management were respectively 4.2% and 38.3% higher than those under conventional management. Large-scale farming was cost-effective in enhancing soil health by improving soil fertility and microbial biomass without increasing soil pollution risks with specific commercial organic compost.
(3) All the biological indicators showed high sensitivity to various management practices. Carbon-related enzymes and saprophytic fungi were more sensitive among all the indicators. Enzyme activity index (EA-index) and microbial community index (MC-index) were more sensitive to management practices. Functional gene index (FG-index) showed a large coefficient of variation, indicating its instability during the determination. Besides, MC-index and EA-index were sensitive to soil health change, since the rate of change in these indices exceeded that of the changes in soil health. MC-index and C-index were the two main explanatory factors and main contributors of carbon cycling, nutrient cycling, and crop productivity functions. These two indices were also highly correlated with soil functions. MC-index and C-index are deeply influenced by factors such as management practices, inherent soil properties, and climatic conditions, which means they were interpretable and informative to soil health. Overall, the MC-index and C-index were sensitive to management practices and could provide a more detailed and mechanistic understanding of soil functions than conventional indicators. With these two indices, soil health and functions can be evaluated effectively by capturing the biochemical processes.
Overall, this thesis constructed an intuitive and effective method for soil health indicators. NA was an effective approach to select indicators, and the SHI-area method could quantify soil health with these indicators. Besides, the integration of microbial community and carbon activity assessments into the soil health indicator system is effective in characterizing soil biological activity. This will support soil health monitoring, assessment, and decisions of sustainable agricultural production in intensified agroecosystems.
