Collapse of beneficial microbial communities and deterioration of soil
health: a cause for reduced crop productivity
Microbial diversity has profound effects on ecosystem functions. Decomposition of organic matter, nutrient cycling, bioremediation of toxics and pollutants, and both the spread and control of infectious
diseases are some key roles of soil microbes in the ecosystems. They determine plant growth. However, the relationships between microbial functioning and sustenance of plant growth are still poorly understood. In the last two decades, there has been a decline in the yields of
major crops in Asia despite the use of recommended cultural practices The reduced crop productivity in conventional agriculture has been explained to have been caused by the indiscriminate use of chemical inputs (fertilizers and agrochemicals) and intensive cropping,
which have made the soil sick . This correspondence illustrates that the collapse of beneficial soil microbial communities under conventional agriculture hampers the delicate positive interactions within and between aboveground and belowground biotic communities, resulting in reduced crop productivity. Conventional tillagesignificantly reduces the diversity of bacteria by reducing both substrate richness and evenness
Therefore, bacterial community assemblages under the tillage have more similar structures than those under zero tillage. However, conservation tillage and legume-based crop rotations in wheat have been observed to support the diversity of soil microbial communities and may affect the sustainability of agroecosystems Plant species identity is a factor influencing microbial community composition. This is because there are strong interactions between aboveground and belowground biotic communities, as revealed by correlations between bacterial metabolic and plant taxonomic diversities Plant species and soil type are two important characteristics affecting the structure of the total bacterial community Soil C/N ratio is most often associated with changes in microbial community composition . Human alteration of soil microbial communities via the alteration of plant community composition and diversity has been observed, which is mediated in part by changes in soil C quality Moreover, 16S rRNA gene and phospholipid fatty acid analyses have revealed shifts in the total microbial community in response to the
different management regimes, indicating that deliberate management of soils can have a considerable impact on microbial
community structure and function in tropical soils . Applications of chemical fertilizers and agrochemicals lower the soil microbial diversity.
Soil bacteria are more sensitive to chemical N fertilizer application during the plant growth cycle It is well known that phytotoxins such
as phenolics, flavonoids and alkaloids are released from plants to the soil. They are known as allelopathic compounds. To make an agroecosystem sustainable these compounds should be removed, a role
which is played by the evolving microbial communities with changing functionality for biodegradation. However, temporal changes in bacterial functionality have been observed to disappear in the conventional cultivation with chemical inputs . This leads to the accumulation of allelopathic compounds, resulting in reduced crop productivity. In contrast, organic farming is generally characterized by elevated microbiological parameters due to incorporation of a wide range of microbial communities via organic manures. Chemical fertilizers and herbicide application in conventional agriculture also affect negatively the potential for topdown control of aboveground pests possibly due to reduced microbial diversity. They also change interactions within and between below- and aboveground components, ultimately promoting negative environmental impacts of agriculture by reducing internal biological cycles and pest control. On the contrary, organic farming fosters microbial and faunal decomposers and this propagates into the
aboveground system via generalist predators, thereby increasing biological control . Land degradation reflected by yield decline, and diseases and pests under conventional agriculture is a common
global problem. As described above, it is mainly caused by the depletion of beneficial microbial communities due to continuous use of chemical inputs and intensive cropping. The degraded lands are rehabilitated using crop rotation or converting into organic farming systems.
These are medium to long-term options with relatively high cost. Handling of organic fertilizers in organic farming is also a difficult job. Therefore, less bulky, short-term and low-cost biofertilizing
methods are now being tested to replenish the beneficial microbial communities in degraded as well as conventional croplands. One such group of biofertlizers is the biofilmed biofertilizers (BBs), which
contain beneficial microbial communities with biofertilizing and biocontroling capabilities . The BBs are now being experimented with rice, tea, maize, etc. in Sri Lanka with initial successes, and
will be shortly tested with wheat and rice in Australia.