How Does Soil Macro-Aggregation Impact on Enzyme Activity and Nitrogen Mineralization?.

Depolymerization of polymeric organic nitrogen (PON) such as protein and chitin, which is mediated by extracellular enzymes, is a rate-limiting step in N mineralization. However, this enzymatic depolymerization might itself be regulated by physical occlusion of the PON substrate resulting from the architectural packing of soil minerals during aggregate formation. To examine the extent to which PON depolymerization is regulated by soil aggregation, we investigated the impact of soil de-aggregation on anaerobic N mineralization rate, extracellular enzyme activities (protease and chitinase) and the relationship between mineralization rate and enzyme activity.

Soils were sampled from grassland (GL) and arable land (AL), air-dried and separated into three fractions (4.75-2, 2-0.25 and 0.25-0.063 mm) by dry-sieving. The fractions were mixed with a proportion of 4:4:1, respectively, to obtain constructed aggregated soils. Corresponding de-aggregated soils were prepared using a mortar and pestle. This procedure mainly disrupted the 4.75-2mm fraction. 

De-aggregation promoted N mineralization rates for both GL (P = 0.016) and AL (P = 0.031) soils. De-aggregation decreased protease activity for GL soils (P = 0.002), while chitinase and total (protease+chitinase) activities were not significantly different between physical treatments (P>0.05). De-aggregation did not influence chitinase, protease and total activities for AL soil (P > 0.05). To examine if this de-aggregation-promoted N mineralization can be attributed to increased bioaccessibility of PON to extracellular enzymes, regression models between N mineralization rates and total enzyme activity for aggregated and de-aggregated treatments were examined. The gradients of both models were positive (p<0.000 and p<0.000, respectively), and the gradient for de-aggregated soils was steeper than that for aggregated soils. This indicated that a greater proportion of the enzymatic potential was realized in de-aggregated soils, enhancing N mineralization rates. As de-aggregation mainly disrupted 4.74-2mm aggregates, our results show how macro-aggregates protect PON from enzymatic attack.