Yield benefits from joint application of manure and inorganic fertilizer in a long-term field pea, wheat and potato crop rotation

Joint fertilization with organic and inorganic fertilizers contributes to sustainable crop production. However, quantitatively understanding the joint fertilization efficacy (JFE) with organic and inorganic fertilizers on yield is limited. This study aimed to investigate the quantitative JFE on crop yield. A 15-year field experiment with pea, wheat, and potato rotation was conducted on a constructed terrace in the semiarid area of the Loess Plateau of China. The fertilizer treatments imposed were (i) inorganic N and P fertilizer (NP); (ii) sheep manure (M); (iii) combined manure plus the inorganic N and P fertilizer (MNP); and (iv) unfertilized control (CK). We defined the yield increase of NP, M and MNP relative to CK (ΔYNP, ΔYM and ΔYMNP), and the JFE was calculated as ΔYMNP−(ΔYM+ΔYNP)ΔYM+ΔYNP×100% When the JFE > 10%, it was considered a synergistic effect; when the JFE was between − 10% and 10%, it was a summing effect, and when the JFE < −10%, it was an offset effect. Across all years, the total crop yield averaged 6127, 3901, 3912, and 1830 kg ha-1 for MNP, M, NP, and CK, respectively. For years 2–8, the yield increases for MNP were 10%− 80% higher than the sum of M and NP (M+NP), showing a synergistic JEF. For years 9–15, the yield increase for MNP was similar or 12%− 30% less than that for M+NP, indicating the summing or offset JFE. By the 15th year, the soil organic carbon (SOC) for CK, NP, M, and MNP increased 30%, 74%, 215% and 185%, respectively, to the initial value. For years 2–8, microbial biomass carbon (MBC), soil inorganic nitrogen (IN) and available phosphorus (AP) in MNP was 20%, 106% and 163% higher than M+NP. A structural equation model analysis indicated that the synergistic JFE was mainly attributable to the rapid increase of AP and IN of MNP during the early years. With the increase in soil fertility and the gradual rise in M yield, the synergistic effect turned into an offset effect. Then the yield of NP decreased due to the soil water deficit, and the offset JEF was turned to summing. The JFE is synergistic in the early years but summing or offset later, showing that joint fertilization is not always increasing yield linearly with soil nutrient input, mainly depending on soil fertility. Clarifying the JFE quantitatively on yield helps optimize fertilization strategies for sustainable farming development.