Root processes counteract the suppression of nitrogen-induced priming effects by enhancing microbial activity and catabolism in greenhouse vegetable production systems

Abstract

Nitrogen (N) fertilization regulates soil organic carbon (SOC) decomposition by altering the priming effect (PE) and root activities, affecting subsequently soil carbon sequestration and crop productivity. However, the effects of long-term N fertilization on the direction and magnitude of SOC and underlying mechanisms priming in the rhizosphere compared with bulk soils remain unclear. In this study, paired rhizosphere and bulk soil samples were collected from a 15-year greenhouse tomato production system under four chemical N fertilizer treatments: 0 (N0), 102 (N1), 327 (N2), and 552 (N3) kg N ha^–1 yr^–1, in addition to uniform manure and straw amendment at 123 kg N ha^-1 yr^-1. These samples were incubated for 49 days with or without the addition of ¹³C-labeled glucose, and the incorporation of glucose-derived ¹³C into CO₂ and phospholipid fatty acids (PLFAs) was monitored to elucidate the mechanisms underlying the PE. The results showed a significant interaction between N fertilization and soil niche. The relative PE was significantly higher under the N0 treatment (1.82–2.02 %) compared with the strongly negative values observed under N1–N3 treatments (-0.81 % to -10.18 %) in both rhizosphere and bulk soils, indicating that increased N availability suppressed SOC decomposition. However, rhizosphere soils exhibited significantly weaker negative PE (-2.66 %) than bulk soils (-4.36 %), primarily due to lower dissolved organic nitrogen (DON) levels and higher microbial abundance and activity, suggesting that rhizosphere processes partially counteracted the suppressive effect of N fertilization. A reduction in relative PE correlated with increases in dissolved organic nitrogen (DON), glucose-derived microbial biomass carbon (¹³MBC), and microbial carbon use efficiency (CUE). Overall, long-term N fertilization suppressed SOC priming by enhancing soil N availability and microbial C assimilation capacity. However, root-mediated microbial legacy effects in the rhizosphere counteracted this suppression, highlighting the importance of N–soil niche interactions in regulating SOC turnover. These findings offer novel insights into soil carbon cycling dynamics and have implications for targeted soil carbon sequestration strategies in intensive greenhouse agriculture.