Responses of soil C, N, and P stoichiometric ratios to N and S additions in a subtropical evergreen broad-leaved forest

Acid deposition from the emission of nitrogen (N) and sulfur (S) has become an important factor affecting the soil nutrient balance and biogeochemical cycling in terrestrial ecosystems. The average levels of N and S deposition in the rainy area of southwestern China from 2008 to 2010 were 9.5 g N m¯² y¯¹ and 19.3 g S m¯² y¯¹, respectively. External additions of N and S fertilizers combined with high levels of acid deposition may affect the soil ecological stoichiometry in the region's widely distributed subtropical evergreen broad-leaved forest. Therefore, we investigated the responses of the soil stoichiometric ratios and enzyme activities to added N (+N), added S (+S), added N and S (+NS), and a control (Ctr) in the 0-20 cm layer in an evergreen broad-leaved forest in the rainy area of southwestern China from April 2013 to April 2015. The results showed that the soil total N (TN) concentration and N/P ratio were higher and the soil organic C (SOC) concentration and C/N ratio were lower in the fertilization treatments than the Ctr, although N and S additions did not significantly alter the soil total P (TP) concentration. The +N, +S, and +NS treatments increased the soil acid phosphatase activity and reduced the soil invertase, cellulase, catalase, and polyphenol oxidase activities. The +N and +NS treatments increased the soil urease activity and reduced soil peroxidase activity. The +S treatment reduced the soil urease activity and did not alter soil peroxidase activity. N and S additions had synergistic decreasing effects on the SOC concentration, C/N ratio, and soil cellulose and catalase activities. Moreover, structural equation models identified that N and S additions regulated the SOC, TN, and TP concentrations via shifting the activities of soil enzymes and the pathways differed between N addition and S addition. In conclusion, N and S additions decreased the SOC concentration, C/N ratio, and most soil C-cycle enzyme activities and increased the TN concentration, N/P ratio, and soil acid phosphatase activity. All these results indicated that external N and S additions combined with acid deposition increased soil N concentrations and exacerbated soil C and P limitations in this forest

The additions of Nitrogen and Sulfur synergistically decrease the release of Carbon and Nitrogen from litter in a subtropical forest

Atmospheric nitrogen (N) and sulfur (S) deposition in subtropical forests has increased rapidly and the current level is very high, thus seriously affecting nutrient (e.g., N and phosphorus (P)) release from litter. However, the specific effects of S addition and its interaction with N on the release of carbon (C), N, and P from litter in subtropical evergreen broadleaved forests are unclear. Therefore, a two-year field experiment was performed using a litterbag method in a subtropical evergreen broadleaved forest in western China to examine the responses of litter decomposition and nutrient release to the control (CK), added N (+N), added S (+S), and added N and S (+NS) treatments. The results showed that the remaining litter mass, lignin, cellulose, C, N, P, and litter N/P ratio were higher, whereas the litter C/N ratio and soil pH were lower in the fertilization treatments than in CK. The annual decomposition coefficients (k-values) in the +N, +S, and +NS treatments were 0.384 ± 0.002, 0.378 ± 0.002, and 0.374 ± 0.001 year−1, respectively, which were significantly lower than the k-values in CK (0.452 ± 0.005 year−1, p < 0.05). The remaining mass, lignin, cellulose, C, and litter N/P ratio were higher, whereas the soil pH was lower in the +NS treatment than in the +N and +S. The interactive effects of N addition and S addition on the remaining litter lignin, cellulose, C, N, and P; the litter C/N, C/P, and N/P ratios; and the soil pH were significant (p < 0.05). In conclusion, the addition of N and S synergistically decreased the degradation of lignin and cellulose and the release of C and N and increased the litter N/P ratio, suggesting that external N and S inputs synergistically slowed the release of C and N from litter and exacerbated litter P limitation during decomposition in this forest

Soil cover improves soil quality in a young walnut forest in the Sichuan Basin, China

The soil quality index (SQI) is based on several key indicators and is used to assess soil quality. More than 250,000 ha of walnut saplings (Juglans regia L.) were planted in previous cropland areas in the Sichuan Basin, China, using a range of soil cover types that may affect soil quality with effects that are unclear. We investigated the effects of white film (WF), black film (BF), shade netting (SN), and maize straw (MS) soil cover types and an uncovered control type (CK) on soil chemical and biological indicators and the SQI in the 0-15 cm soil layer in a young walnut forest in the Sichuan Basin over a 27-month study period. The results showed that all soil cover types increased the soil organic matter (SOM), total potassium (TK), and available potassium (AK) concentrations (p < 0.05), whereas the total nitrogen (TN) and available nitrogen (AN) concentrations were greater only in soils covered by MS than in CK (p < 0.05). The available phosphorus concentrations were 64.1 and 193.2% greater in soils covered by BF and MS treatments, respectively, than in the CK (p < 0.05). The numbers of soil faunal groups (N) were 45.7, 36.4, 37.2, and 101.5% higher in WF, BF, SN, and MS, respectively, than in CK (p < 0.05); the individual numbers (S) were 92.3, 36.2, 100.8, and 154.5% greater in WF, BF, SN, and MS, respectively, than in CK (p < 0.05). The microbial biomass carbon (MBC) was 15.5, 32.3, 45.0, and 77.1% greater in WF, BF, SN, and MS than in CK, respectively (p < 0.05). Redundancy discriminant analysis revealed strong positive interactions between biological indicators (MBC, N, and S) and SOM, AN, and AK concentrations. SOM, TN, AK, S, and MBC were the minimum required variables for the effective assessment of the SQI. All four soil cover types led to an improved SQI (p < 0.05), and MS had the greatest effect on SOM, TN, AN, AP, N, S, MBC, and SQI (p < 0.05). In conclusion, all four soil cover types increased the SOM levels, TK, AK, and MBC concentrations, soil faunal diversity, and SQI. The MS treatment was the most cost-effective and efficient measure to improve soil fertility, ecological function, and overall soil quality in the studied walnut forest

Soil Cover Improves Soil Quality in a Young Walnut Forest in the Sichuan Basin, China

The soil quality index (SQI) is based on several key indicators and is used to assess soil quality. More than 250,000 ha of walnut saplings (Juglans regia L.) were planted in previous cropland areas in the Sichuan Basin, China, using a range of soil cover types that may affect soil quality with effects that are unclear. We investigated the effects of white film (WF), black film (BF), shade netting (SN), and maize straw (MS) soil cover types and an uncovered control type (CK) on soil chemical and biological indicators and the SQI in the 0–15 cm soil layer in a young walnut forest in the Sichuan Basin over a 27-month study period. The results showed that all soil cover types increased the soil organic matter (SOM), total potassium (TK), and available potassium (AK) concentrations (p &lt; 0.05), whereas the total nitrogen (TN) and available nitrogen (AN) concentrations were greater only in soils covered by MS than in CK (p &lt; 0.05). The available phosphorus concentrations were 64.1 and 193.2% greater in soils covered by BF and MS treatments, respectively, than in the CK (p &lt; 0.05). The numbers of soil faunal groups (N) were 45.7, 36.4, 37.2, and 101.5% higher in WF, BF, SN, and MS, respectively, than in CK (p &lt; 0.05); the individual numbers (S) were 92.3, 36.2, 100.8, and 154.5% greater in WF, BF, SN, and MS, respectively, than in CK (p &lt; 0.05). The microbial biomass carbon (MBC) was 15.5, 32.3, 45.0, and 77.1% greater in WF, BF, SN, and MS than in CK, respectively (p &lt; 0.05). Redundancy discriminant analysis revealed strong positive interactions between biological indicators (MBC, N, and S) and SOM, AN, and AK concentrations. SOM, TN, AK, S, and MBC were the minimum required variables for the effective assessment of the SQI. All four soil cover types led to an improved SQI (p &lt; 0.05), and MS had the greatest effect on SOM, TN, AN, AP, N, S, MBC, and SQI (p &lt; 0.05). In conclusion, all four soil cover types increased the SOM levels, TK, AK, and MBC concentrations, soil faunal diversity, and SQI. The MS treatment was the most cost-effective and efficient measure to improve soil fertility, ecological function, and overall soil quality in the studied walnut forest

Simulated nitrogen deposition significantly reduces soil respiration in an evergreen broadleaf forest in western China.

Soil respiration is the second largest terrestrial carbon (C) flux; the responses of soil respiration to nitrogen (N) deposition have far-reaching influences on the global C cycle. N deposition has been documented to significantly affect soil respiration, but the results are conflicting. The response of soil respiration to N deposition gradients remains unclear, especially in ecosystems receiving increasing ambient N depositions. A field experiment was conducted in a natural evergreen broadleaf forest in western China from November 2013 to November 2015 to understand the effects of increasing N deposition on soil respiration. Four levels of N deposition were investigated: control (Ctr, without N added), low N (L, 50 kg N ha-1·a-1), medium N (M, 150 kg N ha-1·a-1), and high N (H, 300 kg N ha-1·a-1). The results show that (1) the mean soil respiration rates in the L, M, and H treatments were 9.13%, 15.8% (P < 0.05) and 22.57% (P < 0.05) lower than that in the Ctr treatment (1.56 ± 0.13 μmol·m-2·s-1), respectively; (2) soil respiration rates showed significant positive exponential and linear relationships with soil temperature and moisture (P < 0.01), respectively. Soil temperature is more important than soil moisture in controlling the soil respiration rate; (3) the Ctr, L, M, and H treatments yielded Q10 values of 2.98, 2.78, 2.65, and 2.63, respectively. N deposition decreased the temperature sensitivity of soil respiration; (4) simulated N deposition also significantly decreased the microbial biomass C and N, fine root biomass, pH and extractable dissolved organic C (P < 0.05). Overall, the results suggest that soil respiration declines in response to N deposition. The decrease in soil respiration caused by simulated N deposition may occur through decreasing the microbial biomass C and N, fine root biomass, pH and extractable dissolved organic C. Ongoing N deposition may have significant impacts on C cycles and increase C sequestration with the increase in global temperature in evergreen broadleaf forests

Responses of soil C, N, and P stoichiometric ratios to N and S additions in a subtropical evergreen broad-leaved forest

Acid deposition from the emission of nitrogen (N) and sulfur (S) has become an important factor affecting the soil nutrient balance and biogeochemical cycling in terrestrial ecosystems. The average levels of N and S deposition in the rainy area of southwestern China from 2008 to 2010 were 9.5 g N m¯² y¯¹ and 19.3 g S m¯² y¯¹, respectively. External additions of N and S fertilizers combined with high levels of acid deposition may affect the soil ecological stoichiometry in the region's widely distributed subtropical evergreen broad-leaved forest. Therefore, we investigated the responses of the soil stoichiometric ratios and enzyme activities to added N (+N), added S (+S), added N and S (+NS), and a control (Ctr) in the 0-20 cm layer in an evergreen broad-leaved forest in the rainy area of southwestern China from April 2013 to April 2015. The results showed that the soil total N (TN) concentration and N/P ratio were higher and the soil organic C (SOC) concentration and C/N ratio were lower in the fertilization treatments than the Ctr, although N and S additions did not significantly alter the soil total P (TP) concentration. The +N, +S, and +NS treatments increased the soil acid phosphatase activity and reduced the soil invertase, cellulase, catalase, and polyphenol oxidase activities. The +N and +NS treatments increased the soil urease activity and reduced soil peroxidase activity. The +S treatment reduced the soil urease activity and did not alter soil peroxidase activity. N and S additions had synergistic decreasing effects on the SOC concentration, C/N ratio, and soil cellulose and catalase activities. Moreover, structural equation models identified that N and S additions regulated the SOC, TN, and TP concentrations via shifting the activities of soil enzymes and the pathways differed between N addition and S addition. In conclusion, N and S additions decreased the SOC concentration, C/N ratio, and most soil C-cycle enzyme activities and increased the TN concentration, N/P ratio, and soil acid phosphatase activity. All these results indicated that external N and S additions combined with acid deposition increased soil N concentrations and exacerbated soil C and P limitations in this forest

Soil cover improves soil quality in a young walnut forest in the Sichuan Basin, China

The soil quality index (SQI) is based on several key indicators and is used to assess soil quality. More than 250,000 ha of walnut saplings (Juglans regia L.) were planted in previous cropland areas in the Sichuan Basin, China, using a range of soil cover types that may affect soil quality with effects that are unclear. We investigated the effects of white film (WF), black film (BF), shade netting (SN), and maize straw (MS) soil cover types and an uncovered control type (CK) on soil chemical and biological indicators and the SQI in the 0-15 cm soil layer in a young walnut forest in the Sichuan Basin over a 27-month study period. The results showed that all soil cover types increased the soil organic matter (SOM), total potassium (TK), and available potassium (AK) concentrations (p < 0.05), whereas the total nitrogen (TN) and available nitrogen (AN) concentrations were greater only in soils covered by MS than in CK (p < 0.05). The available phosphorus concentrations were 64.1 and 193.2% greater in soils covered by BF and MS treatments, respectively, than in the CK (p < 0.05). The numbers of soil faunal groups (N) were 45.7, 36.4, 37.2, and 101.5% higher in WF, BF, SN, and MS, respectively, than in CK (p < 0.05); the individual numbers (S) were 92.3, 36.2, 100.8, and 154.5% greater in WF, BF, SN, and MS, respectively, than in CK (p < 0.05). The microbial biomass carbon (MBC) was 15.5, 32.3, 45.0, and 77.1% greater in WF, BF, SN, and MS than in CK, respectively (p < 0.05). Redundancy discriminant analysis revealed strong positive interactions between biological indicators (MBC, N, and S) and SOM, AN, and AK concentrations. SOM, TN, AK, S, and MBC were the minimum required variables for the effective assessment of the SQI. All four soil cover types led to an improved SQI (p < 0.05), and MS had the greatest effect on SOM, TN, AN, AP, N, S, MBC, and SQI (p < 0.05). In conclusion, all four soil cover types increased the SOM levels, TK, AK, and MBC concentrations, soil faunal diversity, and SQI. The MS treatment was the most cost-effective and efficient measure to improve soil fertility, ecological function, and overall soil quality in the studied walnut forest

Mixed-Species Acacia Plantation Decreases Soil Organic Carbon and Total Nitrogen Concentrations but Favors Species Regeneration and Tree Growth over Monoculture: A Thirty-Three-Year Field Experiment in Southern China

Mixed-species plantations of trees with N-fixing species have the potential of promoting forest productivity and soil fertility. However, few studies in the literature have addressed the advantages of mixed-species plantations of leguminous trees over monocultures of leguminous trees based on in situ inventories over a long time period. Here, we monitored the dynamics of tree community composition, vegetation biomass, soil nutrients, and soil microbial phospholipid fatty acids (PLFAs), in an Acacia mangium monoculture plantation during 33 years of development and compared it with a mixed-species plantation of A. mangium associated with 56 native species which were underplanted 14 years after the initial establishment. Leaf N and phosphorus (P) concentrations of three main species in the overstory and understory of the A. mangium monoculture were measured. Our results showed that the soil organic carbon (SOC), total nitrogen (TN), and available phosphorus (AP) concentrations significantly increased over time during the approximately thirty years of A. mangium monoculture plantation, while the disadvantages were associated with new species regeneration and the increment of vegetation biomass. In the A. mangium monoculture plantation, leaf N concentration of A. mangium, Rhodomyrtus tomentosa, and Dicranopteris dichotoma continuously increased from 21 to 31 years, while the leaf P concentration of A. mangium and R. tomentosa decreased. The mixed-species plantations of A. mangium with native tree species had lower SOC and soil TN concentrations, more new tree species recruitment in the understory, and faster vegetation biomass increment than the A. mangium monoculture. However, the PLFAs of soil microbial groups were slightly different between the two types of plantations. We conclude that improved soil N nutrient condition by A. mangium monoculture benefits N absorption by A. mangium, R. tomentosa, and D. tomentosa, while low soil AP limits P absorption by A. mangium and R. tomentosa. Meanwhile, transforming the A. mangium monoculture into a mixed-species plantation via the introduction of multiple native species into the A. mangium monoculture decreases SOC and TN concentrations but the advantages include improving forest regeneration and maintaining forest growth in a long-term sequence. These findings provide useful and practical suggestions for managing forest monocultures of A. mangium in subtropical regions.ISSN:1999-490

The Responses of Leaf Litter Calcium, Magnesium, and Manganese Dynamics to Simulated Nitrogen Deposition and Reduced Precipitation Vary with Different Decomposition Stages

Litter decomposition is a vital link between material circulation and energy flow in forest ecosystems and is intensely affected by global change factors, such as increased nitrogen (N) deposition and altered precipitation regimes. As essential nutrients, calcium (Ca), magnesium (Mg), and manganese (Mn) play crucial roles in plant energy metabolism, photosynthesis, and membrane transport of plants, and the major source of these nutrients is litter decomposition. However, the dynamics of Ca, Mg, and Mn during decomposition have been largely ignored. Thus, to better understand Ca, Mg, and Mn dynamics during leaf litter decomposition in the scenario of increasing N deposition and decreasing precipitation, we carried out a two-year field litterbag experiment in a natural evergreen broad-leaved forest in the central area of the rainy area of Western China. Two levels of N deposition (ambient N deposition and 150 kg&middot;N&middot;ha&minus;1&middot;y&minus;1) and precipitation reduction (no throughfall reduction and 10% throughfall reduction) were set, i.e., control (Ctr: without nitrogen deposition or throughfall reduction), N deposition (N, 150 kg&middot;N&middot;ha&minus;1&middot;y&minus;1), throughfall reduction (T, 10% throughfall reduction), and N deposition and throughfall reduction (NT, 150 kg&middot;N&middot;ha&minus;1&middot;y&minus;1 and 10% throughfall reduction). We found that leaf litter Ca concentration increased in the early decomposition stage and then decreased, while Mg and Mn concentrations generally decreased during the whole period of decomposition. The amount of Ca showed an accumulation pattern, while Mg and Mn generally showed a release pattern. N deposition and throughfall reduction affected the Ca, Mg, and Mn dynamics, varying with different decomposition stages; i.e., N deposition significantly affected the concentration and amount of Ca, regardless of the decomposition stages, while throughfall reduction significantly affected the Ca concentration in the whole and early decomposition stages. N deposition significantly affected the concentration and amount of Mg in the whole and early decomposition stages, while throughfall reduction had no significant effects. Throughfall reduction significantly affected the concentration and amount of Mn in the whole and late decomposition stages, while N deposition had no significant effects. Ca concentration generally showed a significant positive linear relationship with mass loss in the early decomposition stage; Mg concentration showed a significant positive linear relationship with mass loss in the Ctr and N treatments in the early and late decomposition stages; Mn generally showed a significant negative linear relationship with mass loss, regardless of the decomposition stage. Overall, the results suggest that Ca accumulation is more likely affected by N deposition, while Mg and Mn releases are more likely affected by N deposition combined with throughfall reduction, particularly in the early decomposition stage