Organic fertilizer amendment affect soil aggregates during crop growth: a hotspot for microbial phosphorus turnover?

Farming practices affect soil structure and aggregate formation. The addition of organic fertilizers, such as cow manure, is a practice that can affect soil aggregation and can foster the formation of macroaggregates, which resemble high contents of rather labile soil organic carbon (SOC). Soil aggregates, known to be hotspots for microbial activity, can also be assumed to be hotspots for microbial nutrient cycling. Within the soil system, microorganisms play an active key role in the cycling of phosphorus (P) by: 1) storing P within their biomass, 2) mineralizing non-plant available organic P, and 3) solubilizing inorganic P forms. Microorganisms are thus a key driver in the cycling of P in soil. However, P cycling through the microbial biomass is often limited by SOC availability. The use of organic fertilizers may provide the OC required for microorganisms to cycle P, especially in SOC rich maccroaggregates. The main objective of the present work is to better understand how soil microorganisms’ habitat and P-cycling is affected by the addition of cow manure and how it affect the P cycling through microbial biomass. We collected soils from a long term field trial consisting of different organic soil amendments (Taastrup, Denmark), namely a soil amended with cow manure and a soil amended with mineral fertilizer (nitrogen, potassium and P). We determined soil aggregate size distribution as well as macroaggregate stability, soil C, N and P contents and microbial biomass C, N and P within aggregate size fractions We are able to demonstrate that the application of organic fertilizer has clearly affected soil macroaggregation and stability, as well as the nutrient distribution and content within the aggregates. As large macroaggregates between 2 and 8 mm dominated the sampled soils (49-79 % of the soil mass), we selected these macroaggregates as a functional unit to evaluate the effect of the organic fertilizer on microbial P cycling. We suggest that the addition of cow manure alleviates microorganisms’ OC limitation and thus stimulate P cycling through microbial biomass. The better understanding of soil microorganisms activity and organic fertilizer interaction at the aggregate scale is providing a better understanding of plant-P availability which will benefit the development of future sustainable cropping systems.