How Does Soil Nitrogen Mineralization Respond to Long Term Organic Waste Products Applications? A Multi-Method Evaluation

https://doi.org/10.1111/sum.70253
https://bsssjournals.onlinelibrary.wiley.com/doi/10.1111/sum.70253

Abstract: 

In the long term, repeated applications of organic waste products increase soil organic nitrogen (SON) stocks and would therefore be likely to enhance nitrogen supply to crops. We studied soil nitrogen mineralization in the QualiAgro long-term experiment (northern France), where three urban composts and farmyard manure have been applied biennially since 1998 at 4 tC/ha. In 2014, cropping practices changed with the OWP application rate halved, crop residue incorporation and the replacement of mineral fertilizers with organic fertilizers. The field nitrogen balance method was used to quantify soil N mineralization and compared it with laboratory incubations. We also quantified different indicators linked to soil organic matter (SOM) dynamics (SOM physical and thermal fractionation, enzymatic activity and microbial biomass) which were related to the field N balance results. According to the field nitrogen balance method, the amount of mineralized nitrogen was higher after repeated OWP applications than in the control soil, both before (mean difference of 36% over 15?years) and after (22% over 8?years) the change in management practice. These results were in line with the laboratory incubation results (around 20% of mineralized amount). Concerning the differences between OWP treatments, higher nitrogen stocks are offset by greater organic matter stability, resulting in no significant difference in the amount of mineralized nitrogen across OWP treatments. Mineralization rates (per gram of SON) were higher in soils after repeated OWP applications prior to the change in practice (mean difference of 25% over 15?years) but were similar among treatments thereafter. However, a divergence remained with incubation results, which showed always lower mineralization rates than field N balance results. Among the indicators of SOM dynamics, urease activity normalized to soil organic carbon (SOC) was the best predictor of the N mineralization rate estimated with the field N balance over the entire study period (R?=?0.64). Notably, this indicator correlated mineralization rate much more accurately before (R?=?0.73) the change in management practice than after (R?=?0.41; not significant).

Keywords: Soil organic nitrogen; Mineralization; Organic waste product ; Long-term experiment