Microbial Biomass Carbon Levels Research Articles

The Short-Term Effects of Heavy Thinning on Selected Soil Carbon Pools and Microbial Activity in a Young Aleppo Pine Forest

Pinus halepensis Miller is a widespread tree species in the western Mediterranean basin, where very dense monospecific stands can be found, especially in natural regeneration after forest fires. Silvicultural thinning can reduce the competition of trees for natural resources and favour their development, although its effect depends on the habitat. The present study aims to know the effects on the soil at the physicochemical and microbiological levels after a heavy thinning in a young pine forest stand with a high stocking density. The stand is on a slope where the soil depth tends to decrease with altitude, and shows changes in its physicochemical properties between the upper and lower zones. Several soil carbon fractions (i.e., soil organic carbon (SOC), water-soluble organic carbon (WSOC), and microbial biomass carbon (MBC)), microbial activity (basal soil respiration (BSR)) and enzyme activities (acid phosphatase (AP) and urease (UA)) were analysed at specific dates over a period of about five years after a heavy thinning. The changes in organic matter content were abrupt in the slope, conditioning the observed differences. It is highlighted that the SOC and WSOC contents in the mineral soil were 2.5- and 3.5-fold significantly higher, respectively, in the upper shallow zone compared to the lower deeper zone. This was also reflected in significantly higher levels of gravimetric water content (GWC) and MBC (both about 1.4-fold higher), with higher levels of BSR and UA, and 2.5-fold significantly higher levels of AP. As a result, most of the properties studied showed no significant differences between the thinning treatment and the untreated control. Results varying between dates, with a strong dependence on climate (soil temperature and humidity) of WSOC and UA. It can be concluded that the heavy thinning applied in this short-term case study favoured the growth conditions of the pine without negatively affecting the soil properties studied.

Soil amendment incorporation increases organic carbon by improving soil agglomerate and soil microbial biomass carbon in the alpine grassland

AbstractSoil amendments aiming to enhance soil quality and bolster carbon sequestration have been extensively investigated. However, the specific impacts of diverse soil amendment types on soil total organic carbon content (TOC), soil aggregate and the growth of ryegrass remain largely unexplored, particularly within the unique context of alpine grassland soils in northwest Sichuan. For this, four soil amendments (CK: no soil amendment, CM: cattle manure 2000 kg ha−1, CS: straw amendment 12,000 kg ha−1 and MS: mushroom substrate 18,000 kg ha−1) were applied to alpine grassland soils over a 2‐year duration, conducted in situ during 2017 and 2018, to investigate the influences of these soil amendments on 0–30 cm soil of TOC, total nitrogen (TN), microbial biomass carbon (MBC), soil aggregation, the above‐ground biomass (DMA) and root traits of ryegrass. Compared to CK, the above‐ground biomass exhibited an average of 348.78% in MS, 287.18% in CS and 115.54% in CM, all reaching statistical significance (p < .05). In the topsoil (0–10 cm), the large soil aggregate rate (LSAR > 0.25 mm) showed a significant increase in CM, CS and MS, particularly in 2018, compared to CK. Our findings further indicated that the improvement in alpine grassland LSAR > 0.25 mm was correlated with a rise in TOC by over 69.89% and MBC by more than 27.14%. The MS treatment resulted in a significant increase in above‐ground biomass and TRL (total root length), while also increasing the levels of TN, MBC and soil aggregates (0.25 ~ 0.5 mm) within the 0–10 cm soil. A similar result of CS treatment was observed to increase the total chlorophyll content and RD (root diameter), as well as an increase in SWC and TOC levels. The TN, MBC, TOC and LSAR contributed 44.77%, 20.87%, 6.46% and 6.45% for ryegrass growth. The SEM indicated that soil amendments promote the growth of ryegrass by improving soil agglomerate and increasing MBC, TOC and TN. Our analysis revealed that ryegrass biomass production was limited by soil nutrients in the alpine grassland of northwest Sichuan. The study also highlights the potential impact of soil amendments on future management practices, contributing to a more comprehensive understanding of the subject.

Temporal effects of thinning on soil organic carbon and carbon cycling-related enzyme activities in oak-pine mixed forests

Thinning, a common practice in forest management, has complex effects on soil organic carbon dynamics. In this study, we examined the effects of precommercial thinning on soil organic carbon (SOC) and carbon cycling-related enzyme activities in a thinning chronosequence (4–12 year recovery period) of oak-pine mixed forests in the Qinling Mountains, China. Three treatments were selected, namely, the control (CK) with no thinning, the a4 treatment (thinning 4 years ago), and the a12 treatment (thinning 12 years ago). Soil physicochemical properties, such as SOC, microbial biomass carbon (MBC), easily oxidizable organic carbon (EOC), dissolved organic carbon (DOC), and carbon cycling-related enzyme activities (hydrolase: β-glucosidase [BG], cellobiohydrolase [CBH], and invertase [INV]), were assessed. Our results indicated that thinning significantly reduced SOC by an average of 51.67% in a4 compared to CK, whereas SOC in a12 gradually returned to the unthinned level. The levels of EOC, MBC, BG, and CBH were significantly lower under the a4 and a12 treatments than under the CK treatment. Furthermore, the level of MBC was significantly higher in the a12 treatment than in the a4 treatment. There were no significant differences in DOC or INV among treatments. The structural equation model showed that thinning might regulate SOC by initially decreasing soil water content, MBC, and carbon cycling-related enzyme activities, followed by them gradually recovering to unthinned levels. These findings provide a scientific basis for understanding the response of SOC to forest thinning.

Importance of Dalbergia sissoo for restoration of degraded land: A case study of Indian dry tropical vindhyan regions

To achieve the UN Sustainable Development Goals and preserve biodiversity and ecosystem services, degraded land must be restored. The focus of the current study was to evaluate the impact of Dalbergia sissoo Roxb (D. sissoo), a multipurpose native Indian tree in restoring degraded land. The significance of this species in restoring the fertility of agricultural soil in the dry tropical region of India was also examined in this study. This study showed an improvement in the soil quality (physical, chemical, biological, and enzymatic properties) of rhizosphere zone of D. sissoo (DL) throughout three seasons (rainy, winter and summer) in comparison to D. sissoo based agricultural soil (AL) and degraded (barren) land soil (BL). Soil moisture and water holding capacity (WHC) was significantly higher in DL in all three seasons compared to AL and BL, whereas, lower soil temperature and bulk density were noted in the rhizospheric soil of DL. The amount of soil organic carbon (SOC) was 75–94% higher in DL rhizospheric soil and 12–20% in the AL compared to BL soil in all three seasons. Total nitrogen (TN) and total phosphorus (TP) in the rhizospheric soil of DL were significantly higher (TN, 0.15–0.17% and TP, 16.1–21%) than those in AL (TN, 0.05–0.06%; TP, 11.4–14.2%) and BL ((TN, 0.03–0.04%; TP, 11.6–10.5%) soils, while C: N ratio was higher in BL (13.2–14.8) than AL (10.8–12.1) and lower in DL (6.5–8.2) soil in all three seasons. The rhizosphere soil of DL had considerably greater levels of microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and soil dehydrogenase activity (SDA) than the AL and BL. Pearson's correlation showed a significant strong positive correlation of SOC with WHC, TN, TP, MBC, MBN and soil dehydrogenase activity (SDA) (p < 0.01). Additionally, the principal component analysis (PCA) showed large and positive loading of PC1 for TN, WHC, MBN, MBC, SOC and SDA, suggesting that the restoration efforts should focus on increasing the TN, MBC, MBN, SOC and SDA. Increased SOC, TN, TP, microbial biomass and SDA in the rhizosphere soil of D. sissoo indicate healthy soil conditions in the Vindhyan dry tropical region of India. Therefore, this study advises to use D. sissoo tree extensively in land restoration initiatives for the benefit of both nat ure and people and also to achieve the goal of the United Nations Decade on Ecosystem Restoration (2021−2030).

Organic sulfur mineralization in surface soils from coastal wetlands with different flooding periods affected by the flow-sediment regulation in the Yellow River Delta, China

Soil organic sulfur mineralization can provide absorbable sulfate for plants. However, little is known about the influencing mechanisms of hydrological conditions on soil organic sulfur mineralization in coastal wetlands. Here, an in-situ incubation experiment was conducted to identify the seasonal characteristics of organic sulfur mineralization rates (RNSM) from April to October in surface soils in the seasonal flooding wetlands (SEW), the short-term flooding wetlands (SHW) and the tidal flooding wetlands (TIW) affected by flow-sediment regulation. The results showed lower electrical conductivity (EC) and sand contents and higher water content, and silt and clay contents in SEW, SHW and TIW soils after flow-sediment regulation. The dehydrogenase enzyme activity values in three wetland soils increased from May to August and then decreased after August. Generally, the microbial biomass carbon levels in three wetland soils decreased firstly from May to June and then increased from June to September. However, both arylsulfatase enzyme activity and microbial biomass nitrogen showed different seasonal fluctuations in three wetland soils. The RNSM values in three wetland soils were obviously enhanced during the regulation, but higher RNSM values were observed in SEW soils. Soil properties such as EC and microbial activity were the main factors influencing the RNSM values in these wetland soils. These findings of this study can provide an in-deep understanding of organic sulfur mineralization in wetland soils of the ecotone formed by the interaction between freshwater and seawater in the large river deltas.

Long-term manipulation of mean climatic conditions alters drought effects on C- and N-cycling in an arable soil.

Climate is changing and predicted future scenarios include both changes in long-term mean climatic conditions and intensification of extreme events such as drought. Drought can have a major impact on soil functional processes; soil microorganisms, key to these processes, depend on water and temperature dynamics. Consequently, feedback mechanisms regarding microbially mediated carbon and nitrogen cycling in soils may be affected. There are indications that microbial exposure to increasingly unfavorable environmental conditions influences their stress responses. Here, the long-term field experiment Hohenheim Climate Change (HoCC) provided a research platform to explore how microbial exposure to long-term reduced water availability and soil warming modifies microbially driven soil processes, especially gas fluxes from soil, both during drought and after rewetting. The HoCC experiment is an agroecosystem in which the soil microbiome has been exposed to reduced annual mean precipitation and elevated temperature since 2008. Treatment levels were chosen based on a realistic future climate scenario. In June 2019, we exposed this system to a drought period of four weeks. We found that even after 11years, warming remained a driver of CO2 and N2 O fluxes across the different soil moisture conditions in our drought experiment. Importantly, however, microbial exposure to long-term reduced water availability limited the stimulatory effect of warming on gas fluxes during drought and after rewetting. Our results were neither related to a legacy effect within overall microbial biomass carbon levels nor a shift towards enhanced fungal abundance. We found no indications that extracellular enzyme activities or microbial substrate availability explained the gas flux dynamics observed in our drought experiment. Our study indicates that soil warming promotes gaseous C and N loss even under extreme drought conditions. We suspect, however, that a shift in microbial function following long-term water limitation can hamper the enhancing effect of warming on soil gas fluxes.

Secondary metabolites and anti-microbial/anti-oxidant profiles in Ocimum spp.: Role of soil physico-chemical characteristics as eliciting factors

Ocimum has long been used as a medicinal plant, although little information is available about its bioactive ingredients, and the influence of soil properties on modulation of secondary metabolites in Ocimum has yet to be ascertained. In this study, we present a thorough survey of all potential metabolic compounds in O. sanctum and O. basilicum. In both species, certain compounds (e.g., quercetin, kaempferol, catechin, and S-adenosyl homocysteine) were detected coincidently. In the case of O. basilicum, other vital phenolic acids (e.g., ursolic, vanilic, coumaric, and syringic acids) were identified. The aqueous extracts (AEs) of Ocimum recorded decrease of 6–94% in the proliferation of pathogenic bacteria (e.g., Listeria monocytogenes, Staphylococcus sp., Salmonella sp., and Bacillus sp.). The AEs also showed effective antioxidant activity by reducing free radicals by a factor of 1.04–1.13. Root-zone soil samples of both Ocimum spp. were collected from strategic locations with varying levels of key soil attributes (e.g., soil organic carbon (SOC), microbial biomass carbon (MBC), urease, and phosphatase). At high levels of SOC, MBC, and soil enzymes, the bioactivity of Ocimum spp. was observed to be promoted, especially with respect to secondary metabolite expression, anti-pathogenic activity, and anti-oxidant properties. As such, the findings of strong correlations between secondary metabolite concentrations and bioactivity attributes in Ocimum suggest the potent role of soil quality in eliciting the production of secondary metabolite in association with bioactivity in Ocimum spp.

Elucidating microbial carbon utilization and nitrous oxide dynamics with 13C-substrates and N2O isotopomers in contrasting horticultural soils

Quantifying how cropping systems influence soil C and nutrient turnover is important not only for better understanding the processes that regulate soil functioning, but also for long-term food production goals. We conducted an incubation study on soils collected from adjacent sites with a long-term legacy of annual vegetable, and perennial asparagus, haskap, and apple production. Soils were amended with 13C-labelled vanillin or glucose and incubated for 42 d. Gas and soil samples were collected to measure CO2, N2O, their isotopic signatures, and soil inorganic N and microbial biomass carbon (MBC) dynamics. Throughout the incubation, soil NO3−-N remained highest from vegetable soils, followed by soils from asparagus, haskap, and apple production. In contrast, soils from apple production supported the highest levels of MBC, whereas soil MBC from vegetable production was lowest. In general, glucose-C was utilized more readily than vanillin-C, but differences in glucose-derived CO2-C were more distinct among soils than vanillin-derived CO2-C. Soils from a history of vegetable production had the highest metabolic quotients – indicating poor substrate use efficiency, whereas substrate-C was used most efficiently from soils under apple production. Substrate-C use efficiency inversely corresponded to patterns in denitrifier-N2O production based on 15N2O isotopomers. In general, soils from the intensive vegetable cropping legacy were most susceptible to gaseous losses of both C and N-CO2 by way of soil organic matter priming, and N2O via increased denitrification with additions of labile substrates. Hence, the legacy of past cropping system management had consequences for how C and N was processed in soil.

Effects of Invasive Spartina alterniflora Loisel. and Subsequent Ecological Replacement by Sonneratia apetala Buch.-Ham. on Soil Organic Carbon Fractions and Stock

Background and Objectives: The rapid spread of invasive Spartina alterniflora Loisel. in the mangrove ecosystems of China was reduced using Sonneratia apetala Buch.-Ham. as an ecological replacement. Here, we studied the effects of invasion and ecological replacement using S. apetala on soil organic carbon fractions and stock on Qi’ao Island. Materials and Methods: Seven sites, including unvegetated mudflat and S. alterniflora, rehabilitated mangroves with different ages (one, six, and 10 years) and mature native Kandelia obovata Sheue, Liu, and Yong areas were selected in this study. Samples in the top 50 cm of soil were collected and then different fractions of organic carbon, including the total organic carbon (TOC), particulate organic carbon (POC), soil water dissolved carbon (DOC) and microbial biomass carbon (MBC), and the total carbon stock were measured and calculated. Results: The growth of S. alterniflora and mangroves significantly increased the soil TOC, POC, and MBC levels when compared to the mudflat. S. alterniflora had the highest soil DOC contents at 0–10 cm and 20–30 cm and the one-year restored mangroves had the highest MBC content. S. alterniflora and mangroves both had higher soil total carbon pools than the mudflat. Conclusions: The invasive S. alterniflora and young S. apetala forests had significantly lower soil TOC and POC contents and total organic carbon than the mature K. obovata on Qi’ao Island. These results indicate that ecological replacement methods can enhance long term carbon storage in Spartina-invaded ecosystems and native mangrove species are recommended.

Comprehensive assessment of soil quality for different wetlands in a Chinese delta

AbstractSoil quality is usually considered a powerful tool to reveal soil degradation status and wetland ecosystem health. It is insufficient to reflect actual soil quality based on soil fertility or heavy metal pollution assessment, respectively. A comprehensive assessment was performed to combine soil fertility and heavy metal contamination together, on the basis of 13 soil physical, chemical, biological indicators and six heavy metals, in the Pearl River Delta of China including urban river wetland (URW), ditch wetland (DW), riparian wetland (RiW), and reclaimed wetland (ReW). The moderate fertility level was observed according to the minimum data set, including total nitrogen (TN), microbial biomass carbon (MBC), total phosphorus, total potassium, sand, and salinity. DW soils had significantly higher soil fertility index values than RiW and ReW soils. Low TN, soil organic carbon (SOC), MBC levels, and sandy loam texture revealed potential chemical, biological and physical degradation risks, especially for ReW. In addition, wetland soils were heavily contaminated by heavy metals, particularly for Cd, according to Nemerow Pollution Index. URW soils exhibited significantly higher pollution levels than ReW soils. According to the methodology of comprehensive assessment, soil quality belonged to moderate and low grades because of severe heavy metal pollution, following with moderate nutrition level. Consequently, pollution treatment, fertility (e.g., SOC, N, P, and K), and soil texture improvement are crucial to improving soil quality, preventing further degradation and promoting ecological conservation and sustainability of coastal wetlands in the Pearl River Delta.

Responses of Soil Labile Organic Carbon to a Simulated Hurricane Disturbance in a Tropical Wet Forest

Hurricanes are an important disturbance in the tropics that can alter forest ecosystem properties and processes. To understand the immediate influence of hurricane disturbance on carbon cycling, we examined soil labile organic carbon (LOC) in a Canopy Trimming Experiment (CTE) located in the Luquillo Experimental Forest of Puerto Rico. We trimmed tree canopy and deposited debris (CTDD) on the forest ground of the treatment plots in December 2014, and collected floor mass samples and 0–10 cm soil samples three weeks before the treatment, as well as at scheduled intervals for 120 weeks after the treatment. Within the first week following the CTDD treatment, the mean soil microbial biomass carbon (MBC) and soil LOC in the CTDD plots were significantly greater than in the control plots (soil MBC: 2.56 g/kg versus 1.98 g/kg, soil LOC: 9.16 g/kg versus 6.44 g/kg, respectively), and the mean turnover rates of soil LOC in the CTDD plots were significantly faster than in the control plots. The measured indices fluctuated temporally more in the CTDD plots than in the control plots, especially between the 12th and 84th week after the CTDD treatment. The treatment effect on soil LOC and its turnover rate gradually disappeared after the 84th week following the treatment, while higher levels of soil MBC in the CTDD plots than in the control plots remained high, even at the 120th week. Our data suggest that hurricane disturbance can accelerate the cycling of soil LOC on a short temporal scale of less than two years, but might have a longer lasting effect on soil MBC in a tropical wet forest.

Changes in microbiological attributes of a red latosol under different cropping systems

The objective of this study was to evaluate changes in microbiological attributes of soils under different growing systems. Three cropping systems were evaluated (1) no-tillage system (NTS); (2) conventional tillage system (CTS); and (3) newly scarified no-tillage system (SNTS). The three systems were maintained for 20 years. In addition, a primary forest (PF) fragment was used as a reference. Soil samples were collected at depths of 0.0-0.1 m, 0.1-0.2 m, and 0.2-0.3 m. The following variables were measured: microbial biomass carbon (MBC); soil respirometry (RESP) using the fumigation and incubation method; microbial biomass nitrogen (MBN) using fumigation and extraction; metabolic quotient (qCO2) using the RESP-MBC ratio; and number of spores (NS) of mycorrhizal fungi by plate counting of spores extracted from the soil by wet sieving. Treatments were arranged in a completely randomized block design with five replicates. The different management systems affected the soil microbiota, especially in the superficial layer of 0.0-0.1 m. At a depth of 0.0-0.2 m, the PF presented significantly higher values for all analyzed attributes, except for NS. There were no significant differences in the studied characteristics between the NTS and SNTS at the three depths, indicating that scarification performed only once in the NTS was not sufficient to produce changes in soil microbiological attributes. However, the adoption of the CTS for 20 years promoted a decrease in MBC and MBN. RESP and NS were not significantly different between the cropping systems. Principal component analysis indicated a significant difference in microbiological characteristics between the PF and the areas under management. Therefore, different growing systems change the soil microbiota, and the lower the degree of tilling (NTS and SNTS), the smaller the changes in soil microbiological attributes. Soil preparation practices used in conventional tilling negatively affect soil microbial biomass, decreasing the levels of MBC and MBN.

Exploring the Relationships between Greenhouse Gas Emissions, Yields, and Soil Properties in Cropping Systems

Relationships between greenhouse gas emissions, yields, and soil properties are not well known. Utilizing two datasets from long-term cropping systems in Illinois, USA, our we aim to address these knowledge gaps. The objective of this study was to explore the relationships between the physical and chemical properties and greenhouse gas (GHG) emissions of soil, and cash crop yields over a four-year time-period and following 15 years of treatment implementation in Illinois, USA. The experimental layout was a split-plot arrangement involving rotation and tillage treatments in a randomized complete block design with four replications. The studied crop rotations were continuous corn [Zea mays L.] (CCC), corn-soybean [Glycine max (L.) Merr.] (CS), continuous soybean (SSS), and corn-soybean-wheat [Triticum aestivum L.] (CSW), with each phase being present for every year. The tillage options were chisel tillage (T) and no-tillage (NT). We used an array of multivariate approaches to analyze both of our datasets that included 31 soil properties, GHG emissions (N2O, CO2, and CH4) and cash crop yields. The results from our analyses indicate that N2O emissions are associated with a low soil pH, an increased Al concentration, the presence of soil nitrate throughout the growing season, an increase in plant available water (PAW) and an increased soil C concentration. Likewise, soil CO2 respiration was correlated with low pH, elevated Al concentrations, low Ca, increased PAW, higher levels of microbial biomass carbon (MBC), and lower water aggregate stability (WAS). Emissions of CH4 were associated with increased levels of MBC. Lastly, the yield index (YdI) was correlated with lower levels of soil Ca and available P and lower values of WAS. The association between high YdI and lower WAS can be attributed to tillage, as tillage lowers WAS, but increases yields in highly productive cropping systems in the Midwest.

Long-term irrigation with zinc smelter effluent affects important soil properties and heavy metal content in food crops and soil in Rajasthan, India

ABSTRACTUse of wastewater for irrigating agricultural crops is on the rise, particularly in the developing countries. The present study was undertaken to assess the long-term effect of irrigation with zinc smelter effluent on important soil properties including heavy metal status. Metal concentration in the edible parts of the crops grown on smelter effluent-irrigated soils was also measured. For this purpose, the agricultural lands which have been receiving the zinc smelter effluent irrigation for about five decades at Debari, Udaipur, India were selected. The adjacent tubewell water-irrigated fields were selected as reference. Long-term irrigation with smelter effluent resulted into significant buildup of ethylenediaminetetraacetic acid extractable Zn (57.7 fold), Cu (4.51 fold), Fe (3.35 fold), Mn (1.77 fold), Ni (1.20 fold), Pb (45.1 fold), and Cd (79.2 fold) in soils over tubewell water-irrigated fields. Total Zn, Cu, Mn, Ni, Pb, and Cd content in effluent-irrigated soils was also increased by 27.0, 1.60, 1.40, 1.30, 26.2, and 167 fold, respectively. Risk assessment indicated a very high to moderate potential ecological risk due to Cd, Pb, and Zn in soils close to the immediate vicinity of the smelter plant. Cadmium and Pb concentrations in edible parts of almost all the crops grown on effluent-irrigated soils were above the safe limit of CODEX commission. On an average, soil pH dropped by 0.31 units due to smelter effluent irrigation. Smelter effluent irrigation resulted into significant increase in soil organic carbon, electrical conductivity, and CaCO3 content. On an average, there was decrease in available N (21.0%) and P (20.8%) content in effluent-irrigated soils over the tubewell water-irrigated ones. An increase in available K (102%) and S (26.0 fold) was recorded in effluent-irrigated soils. Long-term irrigation with zinc smelter effluent resulted into reduced microbial activities in soil as evidenced from the level of microbial biomass carbon and dehydrogenase activity. In view of the buildup of heavy metals and subsequent imbalance in essential plant nutrients in smelter effluent-irrigated soils, appropriate remediation-cum-fertilization strategy needs to be adopted for better soil health and plant nutrition.

Soil Organic Carbon Pools as Early Indicators for Soil Organic Matter Stock Changes under Different Tillage Practices in Inland Pacific Northwest

Soil organic matter (SOM) is essential for sustaining soil health and crop productivity. However, changes in SOM stocks in response to agronomic practices are slow and show years later when it is too late for adjustments in management. Identifying early indicators of SOM dynamics will allow early management decisions and quick remedial action. The objectives of this study were to evaluate long-term effects of tillage intensity and timing on SOM pools and determine the most responsive SOM pools to tillage practice. Soil from a long-term (53 yrs) winter wheat (Triticum aestivum L.) - spring pea (Pisum sativum L.) rotation and undisturbed grass pasture (GP) in inland Pacific Northwest (iPNW) was sampled to evaluate the effect of four tillage systems [no-till (NT), disk/chisel (DT/CT), spring plow (SP), and fall plow (FP)] on soil organic carbon (SOC, proxy for SOM), total nitrogen (TN), particulate organic matter carbon (POM-C) and nitrogen (POM-N), permanganate oxidizable carbon (POXC), water extractable organic carbon (WEOC), total dissolved nitrogen (TDN), KCl-extractable nitrogen (KEN), microbial biomass carbon (MBC) and nitrogen (MBN), basal respiration (BR), carbon mineralization (Cmin), and metabolic quotient (qCO2). GP had higher levels of SOC pools than cultivated treatments. On average, tillage significantly decreased SOC and TN by 28% and 26%, respectively, compared to GP. Among the cultivated soils, tillage had no significant effect on SOC and TN, except for DT/CT that had slightly higher SOC than FP (P=0.08). On the contrary, NT and DT/CT significantly (P<0.05) increased levels of POM-C, POM-N, POXC, WEOC, MBC, BR, Cmin, and qCO2 over FP or SP. However, tillage did not affect TDN, MBN, and KEN. The C-pools (POM-C, POXC, MBC, WEOC, BR, and Cmin) were more strongly correlated with SOM than the N-pools (TDN, MBN, and KEN), with an exception to POM-N. Under wheat-pea rotation in the iPNW, reduced tillage systems (NT and DT/CT) have a potential to maintain or increase SOM, which can be assessed early through its physical (POM), chemical (POXC, WEOC), and microbiological (MBC, BR, Cmin) indicators. POXC and WEOC were the most sensitive indicators of tillage-induced changes in SOM dynamics.

Enhanced Nitrogen Availability in Karst Ecosystems by Oxalic Acid Release in the Rhizosphere.

In karst ecosystems, a high level of CaCO3 enhances the stabilization of soil organic matter (SOM) and causes nitrogen (N) and/or phosphorus (P) limitation in plants. Oxalic acid has been suggested to be involved in the nutrient-acquisition strategy of plants because its addition can temporarily relieve nutrient limitation. Therefore, understanding how oxalic acid drives N availability may help support successful vegetation restoration in the karst ecosystems of southwest China. We tested a model suggested by Clarholm et al. (2015) where oxalate reacts with Ca bridges in SOM, thus exposing previously protected areas to enzymatic attacks in a way that releases N for local uptake. We studied the effects of oxalic acid, microbial biomass carbon (MBC), and β-1,4-N-acetylglucosaminidase (NAG) on potential N mineralization rates in rhizosphere soils of four plant species (two shrubs and two trees) in karst areas. The results showed that rhizosphere soils of shrubs grown on formerly deforested land had significantly lower oxalic acid concentrations and NAG activity than that of trees in a 200-year-old forest. The levels of MBC in rhizosphere soils of shrubs were significantly lower than those of trees in the growing season, but the measure of shrubs and trees were similar in the non-growing season; the potential N mineralization rates showed a reverse pattern. Positive relationships were found among oxalic acid, MBC, NAG activity, and potential N mineralization rates for both shrubs and trees. This indicated that oxalic acid, microbes, and NAG may enhance N availability for acquisition by plants. Path analysis showed that oxalic acid enhanced potential N mineralization rates indirectly through inducing microbes and NAG activities. We found that the exudation of oxalic acid clearly provides an important mechanism that allows plants to enhance nutrient acquisition in karst ecosystems.

MICROBIAL CHARACTERISTICS OF SOILS UNDER AN INTEGRATED CROP-LIVESTOCK SYSTEM

Integrated crop-livestock systems (ICLs) are a viable strategy for the recovery and maintenance of soil characteristics. In the present study, an ICL experiment was conducted by the Instituto Agronômico do Paraná in the municipality of Xambre, Parana (PR), Brazil, to evaluate the effects of various grazing intensities. The objective of the present study was to quantify the levels of microbial biomass carbon (MBC) and soil enzymatic activity in an ICL of soybean (summer) and Brachiaria ruziziensis (winter), with B. ruziziensis subjected to various grazing intensities. Treatments consisted of varying pasture heights and grazing intensities (GI): 10, 20, 30, and 40 cm (GI-10, GI-20, GI-30, and GI-40, respectively) and a no grazing (NG) control. The microbial characteristics analysed were MBC, microbial respiration (MR), metabolic quotient (qCO2), the activities of acid phosphatase, β-glucosidase, arylsuphatase, and cellulase, and fluorescein diacetate (FDA) hydrolysis. Following the second grazing cycle, the GI-20 treatment (20-cm - moderate) grazing intensity) contained the highest MBC concentrations and lowest qCO2 concentrations. Following the second soybean cycle, the treatment with the highest grazing intensity (GI-10) contained the lowest MBC concentration. Soil MBC concentrations in the pasture were favoured by the introduction of animals to the system. High grazing intensity (10-cm pasture height) during the pasture cycle may cause a decrease in soil MBC and have a negative effect on the microbial biomass during the succeeding crop. Of all the enzymes analyzed, only arylsuphatase and cellulase activities were altered by ICL management, with differences between the moderate grazing intensity (GI-20) and no grazing (NG) treatments.

Effect of management systems and cover crops on organic matter dynamics of soil under vegetables

Vegetable production in conservation tillage has increased in Brazil, with positive effects on the soil quality. Since management systems alter the quantity and quality of organic matter, this study evaluated the influence of different management systems and cover crops on the organic matter dynamics of a dystrophic Red Latosol under vegetables. The treatments consisted of the combination of three soil tillage systems: no-tillage (NT), reduced tillage (RT) and conventional tillage (CT) and of two cover crops: maize monoculture and maize-mucuna intercrop. Vegetables were grown in the winter and the cover crops in the summer for straw production. The experiment was arranged in a randomized block design with four replications. Soil samples were collected between the crop rows in three layers (0.0-0.05, 0.05-0.10, and 0.10-0.30 m) twice: in October, before planting cover crops for straw, and in July, during vegetable cultivation. The total organic carbon (TOC), microbial biomass carbon (MBC), oxidizable fractions, and the carbon fractions fulvic acid (C FA), humic acid (C HA) and humin (C HUM) were determined. The main changes in these properties occurred in the upper layers (0.0-0.05 and 0.05-0.10 m) where, in general, TOC levels were highest in NT with maize straw. The MBC levels were lowest in CT systems, indicating sensitivity to soil disturbance. Under mucuna, the levels of C HA were lower in RT than NT systems, while the C FA levels were lower in RT than CT. For vegetable production, the C HUM values were lowest in the 0.05-0.10 m layer under CT. With regard to the oxidizable fractions, the tillage systems differed only in the most labile C fractions, with higher levels in NT than CT in the 0.0-0.05 m layer in both summer and winter, with no differences between these systems in the other layers. The cabbage yield was not influenced by the soil management system, but benefited from the mulch production of the preceding maize-mucuna intercrop as cover plant.

Emergence of Diverse Microbes on Application of Biofilmed Biofertilizers to a Maize Growing Soil

Diverse microbial communities in the rhizosphere perform an amazing role in plant growth and productivity. However, conventional agricultural practices such as the use of chemical fertilizer (CF) and tillage have collapsed the diversity of the microbial communities. Direct application of developed fungal-bacterial communities known as biofilmed biofertilizers (BFBFs) to the soil has been introduced recently, and observed to be multi-functional and more effective than conventional biofertilizers. However, the effect of such biofertilizers on soil microbial diversity has not been studied sufficiently world over. Therefore, the current study was carried out to investigate the effects BFBFs on bacterial, cyanobacterial and fungal species in a maize growing soil. A pot experiment was conducted under greenhouse conditions with different fertilizer treatments; 100% chemical fertilizers (CF) recommended for maize, 50% CF, 50% CF + BFBF, and no fertilizer, as the control. Microbial species richness and abundance of bacteria were evaluated after two months of plant growth. Microbial biomass carbon (MBC) was estimated by using chloroform fumigation-extraction technique. Results showed that the species richness of bacteria, fungi and cyanobacteria, and abundance of bacteria were higher in 50% CF + BFBF, compared to 50% and 100% CF. This implies that the action of BFBFs tends to break dormancy of microbial seeds in the soil, resulting in emergence of a diverse microbial community, which may support natural biocontrol of pathogens. An interesting observation was the stimulation of an additional microflora of cyanobacteria by the application of BFBF. MBC levels did not show a significant difference between any of the treatments. We recommend further testing of the BFBFs for increasing microbial diversity and ecosystem functioning and hence sustainability of maize agroecosystems. DOI: http://dx.doi.org/10.4038/cjsbs.v42i2.6612 Ceylon Journal of Science (Bio. Sci.) 42(2): 87-94, 2013

English

A laboratory based pot culture experiment was conducted to investigate the impact of pesticides [endosulfan, dithan-M-45 and 2,4-dichlorophenoxy acetic acid (2,4-D)] at their recommended doses on the changes in microbial biomass carbon; potentiality and proliferation of phosphorus solubilizing organisms; acid and alkaline phosphatase activities; total and available phosphorus in soil. The results of the investigation reveal that though each of the pesticides exerted deleterious effect on phosphorus solubilizing power of soil in the beginning, the overall impact was stimulatory rather than detrimental to the growth and activity of microorganisms associated with the phosphorus transformation in soil. Application of 2,4-D affected the highest level of microbial biomass carbon, growth and activity of phosphorus solubilizing microorganisms and phosphatase activity which was translated to the highest level of available phosphorus in soil. On the other hand, endosulfan brought about the least increment in the level of microbial biomass carbon; potentiality and proliferation of phosphorus solubilizing organisms; acid and alkaline phosphatase activities besides total and available phosphorus in soil. The results of the investigation reveal the safe use of each of the pesticides in the field for the eradication of pest, especially 2,4-D. Key words: Pesticides, microbial biomass carbon, growth and activity of phosphorus solubilizing microorganisms, p-solubilizing power, phosphatases, total and available phosphate.