Shifts in soil nitrogen availability and associated microbial drivers during stand development of Mongolian pine plantations

Abstract Afforestation on degraded lands is an effective measure to control desertification and soil erosion, but these functions are often constrained by soil nitrogen (N) availability. Moreover, soil N availability usually shifts along plantation development, yet we have a limited understanding of factors driving this shift. Here, we examined dynamics of soil N mineralization and nitrification rates along plantation development, and explored mechanisms behind these dynamics from the perspective of N‐cycling microbes. We measured needle litter quality, N‐hydrolyzing enzyme activity, biomass and community composition of soil microbes (phospholipid fatty acid analysis) and ammonia oxidizers (real‐time quantitative PCR and high‐throughput sequencing), and net N mineralization and nitrification rates along a chronosequence of Pinus sylvestris var. mongolica stands with six age classes ranging from 15‐ to 61‐year‐old in the Three‐North region of China. Results showed that stand development increased net N mineralization rate by 54%, nitrate‐N concentration by 106%, and nitrate‐N:ammonium‐N ratio from 1.09 to 2.63. Gram‐positive:gram‐negative bacterial ratio and β‐ N ‐acetylglucosaminidase activity initially increased and subsequently decreased along stand development with the highest values in 40‐year‐old stands, while fungal:bacterial ratio showed the opposite pattern with the lowest value in 40‐year‐old stands. These shifts in soil microbial properties were associated with age‐related changes in needle litter C:N ratio and soil pH. Net N mineralization rate was positively related to β‐ N ‐acetylglucosaminidase activities, but not to soil microbial biomass and community composition. Net nitrification rate was negatively correlated with ammonia‐oxidizing archaeal abundances and positively with ammonia‐oxidizing bacterial abundances. Collectively, our results indicate that soil N availability increases and N cycling accelerates with plantation growth, and suggest that microbial N recycling driven by β‐ N ‐acetylglucosaminidase is tightly related to soil N dynamics along stand development.