Synthetic Nitrogen Fertilizers: Implications for Soil Organic Matter.

Soil organic matter (SOM) is essential for sustaining the food production and maintaining ecosystem services and is a vital resource base for storing N. The impact of long-term use of synthetic-N fertilizer on SOM, however, has been questioned recently. Here we tested the hypothesis if long-term use of synthetic fertilizer-N results in a decrease in SOM. We analyzed peer-reviewed data from 100 long-term field experiments with controlled N fertilizer treatments, representing a wide range of climatic zones, soil types, crops, and management practices.  Using mixed model and meta analysis, concentration and content estimates of SOM were analyzed in terms of response ratios and converted to percent change. Results demonstrate dramatic world-wide declines of 7-16% of SOC and 7-11% of SON with no N amendments. In soils receiving synthetic fertilizer-N, the rate of SOM loss decreased. There was an average increase of 8 and 12% for SOC and SON, respectively, following the application of synthetic fertilizer-N treatments compared to no N-fertilizer applied. Addition of organic matter (i.e. manure) increased SOM, on average, by 37 %. Across the climatic regions, temperature exhibited greater effects on SOM than the amount of organic matter input. When cropping systems fluctuate between flooding and drying, SOM decreased more than in continuous dryland or flooded systems. Although synthetic N-fertilizers showed a lower potential to improve the long-term N-supplying capacity than organic matter, both amendments are vital to increase the intrinsic capacity of agricultural soils to sustain crop productivity. Furthermore, long-term agricultural experiments remain crucial to address fundamental questions on food production and agroecosystem sustainability.