Management effects on soil microbial functions at a South-Indian rural-urban interface

Urbanization leads to effects on many aspects of ecosystems and societies. Among the affected are agricultural systems and, specifically, soils. One of the most evident pathways of urbanization effects on agricultural soils is through an increased demand for agricultural commodities requiring agricultural intensification, hence, changes in management practices that alter soils. In this context, the purpose of this study was to understand how the main agricultural management practices taking place in urbanizing Bangalore affect soil microorganisms, and their functions of soil organic matter decomposition and carbon sequestration. Thus, the first aim of this research was to evaluate the effects of N fertilization level (low and high) and crop type (maize and finger millet) on a set of microbial indicators that characterize the state and functionality of the microbial communities. For this, a two-factorial split-plot design was used, at two fields (irrigated and rainfed) on typical soil types (Nitisol and Acrisol). This study found that more intensified irrigated systems (Nitisol) are more productive, which creates a greater buffer capacity against fertilization- or crop-type effects on soil microbes. In addition, the Nitisol system had larger soil organic C (SOC) levels, microbial biomass and necromass compared with acidic rainfed systems (Acrisol). Whereas microbial biomass remained similarly active in both systems. Soil pH and the amount of particulate organic matter (POM), along with site-condition differences (clay content, irrigation), rather than N-fertilization level, were major drivers of microbial parameters. These results pointed out the need for improving SOC stocks and nutrient balances by providing fresh organic inputs to the fields, especially under rainfed agriculture and to the need of liming and evaluate liming impacts on SOC-associated microbial functions in Bangalore´s soils. Thus, the second aim of this research was to address the effect of liming on an integrative microbial indicator of microbial C-cycling and C sequestration in soils: microbial carbon use efficiency (CUE). The effect of pH and liming were evaluated on total CUE (including microbial residues), CUE of microbial biomass (CUEMB) and fungal biomass after maize-litter addition in a 6-week incubation experiment. Microbial measurements were addressed in both soils, on the acidic Acrisol (limed and unlimed) compared to the Nitisol (unlimed). Litter addition benefited fungal biomass, which was at the end greater in the limed Acrisol than the Nitisol and was positively associated to CUEMB. An increased pH decreased CUE and promoted positive priming of SOC. According to these results is the low input of plant residues, and not reduced microbial efficiency, the most likely cause of lower SOC levels in the Acrisol. The last aim of this research was to relate these field-scale and experimental findings to actual SOC dynamics taking place in Bangalore at the regional scale. This aim was split into two steps. The first step was to identify the medium- and long-term effect of relevant management practices on SOC levels, conducting a review of literature about effect of management practices on Indian soils (and Bangalore soils when available) followed by a local meta-analysis of N-fertilization effects in Bangalore. The second step was to understand how the socio-economic effects of rapid urbanization across Bangalore´s rural-urban interface could be leading to changes in such management practices. This was evaluated using interview data from farmers’ households across an urbanity gradient in Bangalore. Despite of the mild effect of increased N-fertilization on microbial parameters found in the current research, at the broader scale, N fertilization, in a recommended dose, positively influences SOC levels in Bangalore´s soils, especially when mineral and farmyard manure (FYM) fertilization are combined. However, this combination was less common than single mineral fertilization in Bangalore. Furthermore, conservation practices are necessary in Bangalore to improve current low SOC levels but less than 50% of interviewed farmers apply conservation practices such as minimum/non-tillage and only 16% applied residue management such as mulching, crop-residue application or the use of cover crops. Residue management practices are significantly associated to market integration and to cultivation of specific irrigated (flowers, vegetables, fruits) and non-irrigated (pulses) crops. While urbanization reduces minimum-tillage application. When farmers rely more on agriculture, have the means and are better integrated to markets, it is more likely that they adopt intensification practices like irrigation. This practice is strongly associated with crop choice, i.e., adoption of irrigated crops like fruits, vegetables and fodder instead of traditionally non-irrigated crops like cereals. Overall, our study points out that improved management of crop residues and cultivation of diverse crops, application of liming and increasing farmer economic opportunities (e.g., off-farm income, durable assets, market integration) would improve the current status of Bangalore´s soils.