by Steve Wood, Columbia University
It is almost trivial to state the importance of fertilizers to the incredible increase in global food production over the last half century. Despite this agronomic revolution, environmental scientists have shown severe environmental damages associated with high nutrient input when excess nutrients drastically modify biogeochemical processes and species composition. Scientists are now trying to figure out how to keep up agricultural growth while minimizing environmental damages.
But fertilizers may also pose a threat to agriculture itself. Agriculture depends on short-term nutrient input but also on soil properties that change over longer time scales, such as soil organic matter, which plays a key role in soil structure, water holding capacity, and nutrient availability to plants. Yet the microorganisms that control the build up and break down of organic matter are highly sensitive to fertilization. If agronomic nutrient inputs drastically modify the functional capacity of soil microbial communities, there may be long-term effects on agronomic capacity that aren’t foreseeable under present trajectories of agronomic growth.
These potential effects are particularly potent on smallholder farms in sub-Saharan Africa that are traditionally low in external nutrient inputs. A surge in global interest in food security has led huge investment in agricultural development projects aimed to increase crop productivity on smallholder African farms. This effort is often called the African Green Revolution and one of its key components of this strategy is the increased use of mineral nutrient inputs.
Our work aims to quantify how agriculture development-driven changes in nutrient budgets on smallholder African farms are impacting the functional capacity of soil microbial communities. In a first set of studies, we collected soil from both experimental fertilizer addition plots and actively managed farms in western Kenya. We classified actively managed farms as low fertilizer, high fertilizer, or high fertilizer plus the incorporation of seasonal fallows that build up soil organic matter. We measured the taxonomic composition of microbial communities, the abundance of key functional genes, and the ability of the microbial community to use a range of carbon substrates.
We found that the taxonomic composition of microbial communities is highly sensitive to mineral fertilizer addition, as has been shown elsewhere. But these changes in taxonomic composition are not associated with losses in the functional capacity of microbial communities. Instead, microbial functional capacity was significantly greater when mineral fertilizers were combined with organic inputs. The abundances of a range of functional genes involved in C and N cycling were significantly elevated as was the ability of microbes to synthesize carbon substrates.
This pairing of mineral fertilizers and organic matter is also responsible for the greatest increase in crop growth, suggesting that there might be important synergies between increasing crop yields and maintaining the functional capacity of soil microbial communities.
Two recent papers from this project:
Wood SA, Bradford MA, Gilbert JA, McGuire KL, Palm CA, Tully KL, Zhou J, Naeem S (2015) Agricultural intensification and the functional capacity of soil microbes on smallholder African farms. Journal of Applied Ecology.
Wood SA, Almaraz M, Bradford MA, McGuire KL, Neill C, Naeem S, Palm CA, Tully KL, Zhou J (2015) Farm management, not soil microbial diversity, controls nutrient loss from tropical smallholder agriculture. Frontiers in Terrestrial Microbiology, 6, doi:10.3389/fmicb.2015.00090.
More information about Steve Wood’s research can be found here: www.stephenwoodecology.com