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In order to explore the effects of enzyme activities from plant root exudates on rhizosphere soil enzyme activities and nutrients in karst areas, the activities of β-1,4-glucosidase (βG), β-N-acetyl-glucosaminidase (NAG), leucine aminopeptidase (LAP), and acidic phosphatase (ACP) of fine root exudates and rhizosphere soils in four vegetation restoration stages, which are shrub-grassland, shrubland, shrub-arbor forest and arbor forest, were measured. And, their relationships with soil carbon (C), nitrogen (N), phosphorus (P) contents were analyzed. The results were as follows: (1) The four enzyme activities of rhizosphere soils and root exudates were significantly higher in the late stage of vegetation restoration than that in the early stage of vegetation restoration. The enzyme activities C∶P to N∶P ratios of root exudates in arbor forest were significantly higher than those of other three stages, while the two parameters of rhizosphere soils were opposite. (2) Correlation analysis showed that the enzyme activities of root exudates were positively correlated with the corresponding soil enzyme activity. Compared with the enzyme activities of root exudates, the correlation between soil enzyme activity and related nutrients became significantly stronger. In addition, the enzyme activities of βG, NAG and LAP in rhizosphere soils and root exudates were positively correlated with soil organic carbon (SOC) and total nitrogen (TN) in rhizosphere soil, while the enzyme activities of ACP in rhizosphere soils and root exudates were positively correlated with available phosphorus (AP) in rhizosphere soil. The above results indicate that vegetation restoration has positive effects on increasing enzyme activities of root exudates and soils. The enzymes of root exudates are the indispensable supplements for soils, and play an important role in promoting carbon, nitrogen and phosphorus nutrient cycling. To sum up, it will be that the regulation of root exudates and its enzyme activities may provide a new perspective for vegetation restoration in karst ecosystem.

The ecological stoichiometry of fine root carbon (C), nitrogen (N), and phosphorus (P) is considered to reflect the strategies whereby plants adapt to the soil environment. In order to gain an understanding of the seasonal variation in fine root C∶N∶P ratios in different forest types of karst ecosystem, we measured the C, N, and P contents and ratios of living and dead fine roots in shrubland and arbor forest, as well as the influence of abiotic and biotic factors. The results were as follows: (1) C, N, and P contents in the fine roots of arbor forest were higher than those of shrubland, thereby indicating that the fine roots of arbors may by characterized by a stronger absorption and storage of nutrients than those of shrubs. In addition, the contents of C in living fine roots were significantly higher than those in dead fine roots (P

Phosphorous (P) limitation is common not only in tropical rainforest and savanna ecosystems, but also in karst forest ecosystems. Soil phoD-harboring microorganisms are essential in soil P cycles, but very little information is available about them in karst ecosystems. A total of 36 soil samples were collected from two types of forest ecosystems (karst and non-karst) over two seasons (rainy and dry), and the diversity and community structure of soil phoD-harboring microorganisms were measured. The contents of available P (AP), soil total P (TP), microbial biomass P (MBP) and the activity of alkaline phosphatase (ALP) in karst forest soils were higher than those in non-karst forest soils, whereas the contents of CaCl2-P, citrate-P, enzyme-P and the activity of acid phosphatase (ACP) were the opposite. Soil AP content was significantly higher in the rainy season than in the dry season, whereas ALP activity was the opposite. The community structure of phoD-harboring microorganisms was more influenced by forest-type than season. The network connectivity was higher in non-karst forests than in karst forests. Two dominant orders, Burkholderiales and Rhizobiales, were the keystone taxa in these networks in two forests, and their relative abundances were higher in non-karst forests than in karst forests. The microorganic diversity indices (e.g., Shannon–Wiener, Evenness, Richness, and Chao1) were substantially higher in karst than in non-karst forests. These indices were positively correlated with the contents of SOC and TN in the two forests; meanwhile, richness and evenness indices were positively correlated with citrate-P, HCl-P, and TP in non-karst forests. Structural equation modelling results showed that the relative abundance of phoD-harboring microorganisms was mainly influenced by pH and AP, with direct affection of soil AP, pH, and ALP activity, and indirect affection of ALP activity through affecting AP. These findings highlight that the P cycle is mainly regulated by the diversity of phoD-harboring microorganisms in karst forest ecosystems, whereas it is mainly regulated by dominant taxa in non-karst forest ecosystems. In future, regulating the interaction networks and keystone taxa of phoD-harboring microorganisms may be critical to alleviating P limitations in karst forest ecosystems. [ABSTRACT FROM AUTHOR]

Understanding soil nitrogen (N) dynamic and availability during afforestation (the conversion of cropland to forest plantations) is critical to maintain forest growth and long-term productivity, especially in rainfall-rich, subtropical region. However, only few studies have investigated the inherent N transformation processes involved in N availability in subtropical acidic soils. In a 15 N tracing study, nine soils from croplands, 10-y and 50-y afforested woodlands were sampled to investigate the changes in soil gross N transformation rates in humid subtropical China. Gross N transformation rates were not significantly different in soils under 10- and 50-y after afforestation. Compared to cropland, however, afforestation stimulated the rates of mineralization, microbial NH 4 + immobilization and adsorption of NH 4 + , leading to a faster turnover of NH 4 + pool in afforested soils. Moreover, afforestation inhibited autotrophic nitrification and resulted in NO 3 − production dominated by heterotrophic nitrification. Furthermore, afforestation significantly enhanced NO 3 − consumption mainly through the increase in microbial NO 3 − immobilization rather than dissimilatory NO 3 − reduction to NH 4 + in soil. These differences in gross N transformation rates resulted in low net NO 3 − production and strong NO 3 − retention capacity in afforested soils, similar to that found in undisturbed natural forest. Our results suggest a rapid recovery (several years) of soil N retention following afforestation. Soil NO 3 − retention capacity was correlated positively with TOC, TN, WHC, CEC, Al, free Al oxide and exchangeable Al 3+ , but negatively with pH, Ca and exchangeable Ca 2+ , indicating that the absence of agricultural management (e.g., N fertilizer and liming) and alteration in soil environment by tree establishment are responsible for the recovery of N retention capacity in subtropical acidic soils during afforestation. [ABSTRACT FROM AUTHOR]

Soil ammonia oxidizers play a critical role in nitrogen cycling and ecological restoration. The composition and structure of soil ammonia oxidizers and their impacting factors were studied in four typical ecosystem soils, tussock (T), shrub (S), secondary forest (SF), and primary forest (PF), during vegetation restoration in the Karst region of Southwest China. The composition and structure of the ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) communities were characterized by sequencing the amoA and arch- amoA genes, respectively. The diversity of soil ammonia oxidizers (except in S) and plant Shannon diversity index gradually increased with vegetation restoration, and the ammonia oxidizer communities differed significantly ( p < 0.001). Amplicons of AOA from the Nitrososphaera cluster dominated all four ecosystem soils. AOB Nitrosospira cluster 3b only appeared in PF and SF soils, while Nitrosospira cluster 3a species were found in all soils. Changes in AOB paralleled the changes in soil ammonium content that occurred with vegetation restoration. Redundancy analysis showed that the distribution of dominant AOB species was linked to pH, soil urease activity, and soil C/N ratio, whereas the distribution of dominant AOA species was mainly influenced by litter nitrogen content and C/N ratio. These results suggested that the composition and structure of the AOB community were more sensitive to changes in vegetation and soil ammonium content, and may be an important indicator of nitrogen availability in Karst ecosystem soils. [ABSTRACT FROM AUTHOR]