Organic fertilization strengthens multiple internal pathways for soil mineral nitrogen production: evidence from the meta-analysis of long-term field trials

Organic carbon based amendments can improve soil structure and fertility, as well as increase composition and diversity of soil microbial community. One of the major functions of improving soil fertility is to achieve effective nitrogen (N) management and mineralization. In this study, 746 paired observations were pooled from 20 long-term field experiments to verify the hypothesis that compared to synthetic only fertilization, long-term application of organic fertilization alone or in combination with synthetic fertilization could strengthen multiple internal pathways for soil N supply under various climatic conditions. We found that long-term application of synthetic N fertilizers alone or in combination with phosphorus (P) and potassium (K) led to an increase only in the recalcitrant organic N mineralization rate (M_Nrec) by 210% and 263%, respectively. However, long-term application of organic fertilizers alone or in combination with synthetic N fertilizers increased M_Nrec, labile organic N mineralization rate (M_Nlab) and release of adsorbed ammonium from cation exchange sites (R_NH4a) by 160% and 200%, 153% and 353%, and 1025% and 541%, respectively. This indicates that organic fertilization can strengthen multiple internal pathways for mineral N production. The long-term co-application of organic and synthetic fertilizers stimulated M_Nlab (197%), M_Nrec (151%) and R_NH4a (563%) in subtropical regions, but it had no effect on heterotrophic nitrification (O_Nrec). In contrast, it stimulated M_Nrec (505%), R_NH4a (633%) and O_Nrec (184%) in temperate regions, with no observed effect on M_Nlab. These data confirm that if one N supply source is shut-off under specific climatic conditions the other pathways continue to provide N. The meta-analysis advances our understanding of agroecosystems and as such help to improve frameworks for enhancing soil health.