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Increased nitrogen (N) deposition endangers the biodiversity and stability of forest ecosystems, and much of the original phosphorus (P) parent material continues to decrease in most lowland tropical forests. It remains poorly understood as to how soil microbial diversity at a molecular level responds to the addition of excess N and mitigation of soil P limitation, as well as their influencing factors, in the N-rich tropical forest ecosystems. To reach a better understanding, we conducted a six-year N and P-addition experiment consisting of three treatments: N-addition (150 kg N ha−1 yr−1), P-addition (150 kg P ha−1 yr−1), and NP-addition (150 kg N ha−1 yr−1 plus 150 kg P ha−1 yr−1), besides a control treatment in an old-growth tropical forest in southern China. We examined the snapshot responses of soil bacterial richness and community composition to the elevated N and P levels after six years using a 16S rRNA gene MiSeq sequencing method. The soil bacterial α-diversity, which is represented by Chao1 index in terms of bacterial richness, was 783 ± 87 (mean ± SD) across all samples in this study. The N addition caused a decline in soil bacterial richness, most likely through its negative effect on soil pH. The decrease in soil pH resulted from the direct N input and indirect NO3− increase. However, the P treatment had no effect on soil bacterial richness. The NP treatment also reduced the soil bacterial richness as the N addition. These results suggested that the P input could not alleviate the loss of soil bacterial richness induced by excess N deposition in the old-growth N-rich tropical forest. The Acidobacteria, which comprised 31.1% of the soil bacterial community, were the most dominant bacteria across all samples. The addition of P shifted the soil bacterial community composition. The elevated P availability with P-addition and the decreased understory plant coverage in the N-input treatment altered the soil bacterial β-diversity. Our results highlight the different roles of N and P depositions in shaping the soil bacterial richness and community composition, thereby causing concomitant changes in understory plant and underground microbial communities in this ecosystem.

In southern China, high levels of atmospheric nitrogen (N) are being deposited in forests. Soil acidification and high rates of soil nitrification and subsequent NO 3 − leaching have been observed in the N-saturated forests. We previously did not detect NH 3 -oxidizing bacteria in non-N-saturated or N-saturated forest soils. However, NH 3 -oxidizing archaea were present in both soils, more in the saturated ones. The purpose of this study was to investigate the roles of autotrophic NH 3 -oxidizing archaea and heterotrophic nitrifiers in soil N transformations in N-saturated and non-N-saturated forests in southern China. We investigated the contribution of heterotrophic nitrifiers in the soils by determining the gross nitrification rates with and without an inhibitor of autotrophic nitrification, acetylene (C 2 H 2 ). We also reevaluated nitrification by NH 3 -oxidizing archaea by correlating the C 2 H 2 -inhibited gross nitrification rates with the abundance of the amoA transcripts of NH 3 -oxidizing archaea. We further measured the gross NH 4 + production rates and analyzed the community composition of the NH 3 -oxidizing archaea. The results suggest that NH 3 -oxidizing archaea, rather than heterotrophic nitrifiers and NH 3 -oxidizing bacteria, are responsible for the nitrification in the N-saturated forest soils. NH 3 -oxidizing archaea in the soils could be acidophilic, having low amoA diversity, indicating their strong adaptation to the highly acidified soils. The gross NH 4 + production rate did not differ between N-saturated and non-N-saturated forests; however, the gross nitrification rate was higher in N-saturated forests. Consequently, the high abundance and NH 3 oxidation activity of NH 3 -oxidizing archaea caused the high rates of nitrification and subsequent leaching of NO 3 − in the N-saturated forest. This study suggests that acidophilic NH 3 -oxidizing archaea have a great impact on soil N cycling in N-saturated forests.

Intensified anthropogenic activities will increase rates of nitrogen (N) deposition over the next decades, especially in the tropics. There are urgent needs to know how soil microbial community in N-rich tropical forests responds to long-term N deposition. This study examined effects of long-term N additions on soil microbial biomass (determined by chloroform fumigation), microbial community composition (based on phospholipid fatty acids, PLFAs), and microbial enzyme activities, using an ongoing experimental N additions field in an N-rich tropical forest of South China. There were four N additions levels: no additions (Control); 50 kg N ha−1 yr−1 (Low-N); 100 kg N ha−1 yr−1 (Medium-N), and 150 kg N ha−1 yr−1 (High-N). Results showed that long-term N additions significantly decreased microbial biomass carbon (MBC) and nitrogen (MBN), but had little effects on total PLFAs. However, elevated N inputs significantly reduced the relative abundance of bacterial PLFAs, especially gram-negative bacterial PLFAs with higher gram-positive bacteria: gram-negative bacteria ratio in N treatment plots. Although N additions did not change fungi: bacteria ratio, the proportion of arbuscular mycorrhizal fungi increased significantly with N additions. Long-term N additions greatly increased bacterial stress indexes and enhanced specific enzyme activity (activity per unit of microbial biomass) involved in carbon, nitrogen and phosphorus mineralization. Meanwhile, shifts in microbial community composition and specific enzyme activity were correlated well with soil pH and available N. These results suggest that N-mediated environmental stresses can play an important role in shaping microbial community, and that soil microbes will invest more resources on enzyme production in N-rich forest under elevated N deposition.

The ratio of β-1,4-glucosidase (BG) to β-1,4-N-acetylglucosaminidase (NAG) activity (BG:NAG ratio) is often used as an indicator of the relative resource allocation of soil microbes to C acquisition compared with N (the ecoenzymatic stoichiometry hypothesis). An increasing number of recent studies have used this index to assess the nutrient status of microbes. However, the validity of this index for assessing the nutrient status of microbes is not well tested. In this study, we collected published data and tested that validity by investigating whether N fertilization elevated the BG:NAG ratio, to test the assumption derived from the ecoenzymatic stoichiometry hypothesis that microbes reduce their allocation to the N-acquiring enzyme (NAG) under N-enriched conditions. Of the observations, 54% (82/151) did not support the hypothesis because those studies showed lower BG:NAG ratios in N-enriched soils than under ambient conditions (i.e., N was not enriched), especially when the ambient BG:NAG ratio was higher than 2.0 (77%, 59/77). This suggests that the BG:NAG ratio does not necessarily indicate the microbial status for C or N limitation. Rather, we hypothesized that NAG targets chitin or peptidoglycan, which accumulate at later stages of decomposition and that the variation in BG:NAG explains the decomposition stage. A negative correlation of BG:NAG ratio with polyphenol oxidase activity, which generally increases with decomposition, supported our hypothesis.

Dissolved organic matter (DOM) drives many fundamental biogeochemical processes (e.g., carbon storage, nutrient cycling, and soil development) in forest soil. However, the molecular-level characteristics of DOM derived from different types of tropical forest soils are poorly understood. Here, water samples at different soil depths (0, 20, and 40 cm) from tropical legume (Acacia auriculiformis, AA) and non-legume (Eucalyptus urophylla, EU) tree plantations were analyzed using absorption and fluorescence spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and solution-state 1H nuclear magnetic resonance (NMR) spectroscopy. The FT-ICR MS results indicated that DOM persisted in the soil, but its molecular composition notably shifted from low-mass (150–300 Da) and more-aromatic molecules to middle- (300–450 Da) and high-mass (>450 Da) and less-aromatic molecules with increasing soil depth. This was primarily mediated by consumption and mineral sorption of low-mass plant-derived DOM (e.g., low-mass carbohydrates and polyphenols) and further formation of larger microbial products (e.g., protein-like and lipid-like compounds). In addition, a higher abundance of microbial-derived molecules (e.g., protein-like and carboxyl-rich alicyclic molecules) was found at the legume plantation relative to the non-legume plantation, which suggests a faster microbial turnover of DOM. Also, the legume plantation had greater enrichment of middle- and high-mass and condensed aromatic-like DOM components in soils. These findings improve our understanding of the drivers that mediate the response of DOM to soil depth and tree species in tropical plantations.