Effects of Biochar and Maize Roots on CO 2 Emissions and Microbial Biomass C in Loess Soils

Microbial biomass (MB) plays a critical role in residue decomposition and soil organic matter (SOM) turnover. We investigated the effects of the initial size of soil MB pool and nitrogen (N) availability on the incorporation of ryegrass residue carbon (C) into microbial biomass C (MBC) and dissolved organic C (DOC). Soils from a row crop system (CS) and an adjacent grass sod (GS) were preincubated with (i.e., increased MB) and without (i.e., unchanged MB) glucose to produce soils with two levels of initial MB size before residue additions. Ammonium sulfate was added to test the effect of N availability. Residue addition increased DOC production in both soils, regardless of initial MB size and N availability, contributing 9 to ∼22% to the total pool. Residue quality was observed to affect the incorporation of residue C into DOC, which depended on soil type and initial MB size. However, the assimilation of residue C into MBC was not affected by the quality of ryegrass residue. Compared with the control (which did not have residue and N additions), a significant increase in SOM‐derived MBC by residue addition was observed in CS without glucose preincubation but not in the glucose preincubated CS and in GS. This indicated that the primed MBC by residue addition depended on initial MB size and soil type. The assimilation of residue C into MB was marginally inhibited by the preincubation with glucose in GS but was promoted in CS. With the addition of extra N, higher ryegrass‐derived MBC was observed in CS with glucose preincubation compared with the corresponding treatments without preincubation, which was not found in GS. These results suggested that residue‐derived MBC was not only regulated by initial MB size and N availability but also was affected by soil management history. However, N addition reduced ryegrass‐derived DOC production in GS without glucose preincubation. Apparently, both soil MB size and N availability largely affected the assimilation and incorporation of residue C in soil labile pools (i.e., DOC and MBC), and the extent of this relationship varied between two agricultural soils. Core Ideas Ryegrass residue addition primed the production of SOM‐derived DOC and MBC. Residue quality affected ryegrass‐derived DOC in crop soils. N addition reduced ryegrass‐derived DOC in grass soils. N addition increased ryegrass‐derived MBC in crop soils. Initial MB and N availability regulated the incorporation of residue C into DOC and MBC.

Effects of straw biochar on microbial-derived carbon: A global meta-analysis

Increasing soil carbon (C) sequestration in paddy fields is significant for ensuring food security and achieving C neutrality in China. Biochar has been widely used as a soil amendment; still, long-term effects on the mechanisms of biochar's effect on soil C accumulation and the mediating role of microorganisms are poorly understood. To address this issue, three field experiments on paddies were chosen (Changsha, Nanjing, and Jiaxing), where biochar was applied for 7 to 8 years. The treatments included control (no addition), N (N fertilizer, 120 kg ha-1), N+B1 (N and low amount of biochar, 15-24 t ha-1), and NB2 (N and high amount of biochar, 22.5-48 t ha-1), effects on soil organic C (SOC) mineralization, dissolved organic C (DOC), activities of enzymes, microbial biomass C (MBC) and community composition (based on phospholipid fatty acids (PLFAs), and C utilization efficiency (CUE) were studied. Biochar reduced cumulative CO2 emissions in Changsha (by 32-34 %), Jiaxing (3.0-27 %) (p<0.05), and in Nanjing with under NB2 treatment (by 36 %) compared to N treatment. Biochar increased soil pH (0.03-0.38 units) in Changsha and Nanjing but did not affect Jiaxing plots. Biochar increased SOC, total N, chitinase activity, MBC (by 18-28 %,) and CUE (by 24-65 %, except in Jiaxing) but decreased DOC content (by 3-14 %) and peroxidase activity. Biochar addition increased the total and bacterial PLFA contents and decreased the bacteria:fungi ratio at the three sites (except for total PLFA in Nanjing) compared to the N treatment. The correlation analysis revealed that cumulative CO2 emission was reduced under the increase of pH, MBC, SOC, and CUE, bacterial PLFA, and stimulated by DOC content and the rise of bacterial:fungi ratio. These indicated that long-term biochar amendments mainly increased the amount of C that bacteria can assimilate; the increase of MBC content and CUE could point to the stimulation of microbial C sequestration.

Conversion from rangeland to cropland potentially influences soil organic matter (SOM) dynamics and biochemical properties. The purpose of the current study was to assess changes in soil properties and investigate the interaction between SOM and enzyme activities following cultivation of native rangelands for more than 40 years. The quantities of soil organic carbon (C) and microbial biomass C (MBC), aggregate stability, microbial respiration rate (MRR), potential C mineralization (PCM) and the activities of soil enzymes involved in C, N, P and S cycling at the 0–20 and 20–40 cm depths were evaluated in never-cultivated natural rangelands and their corresponding farmlands with similar soil type and climatic conditions at Dehnow located in Central west, Iran. Cultivation in native rangelands resulted in a significant decline of most soil properties in farmlands, depending on soil sampling depths. Conversion of rangelands to their farmland counterparts decreased soil C (50–66 %), MBC (48–61 %), MRR (20–44 %), PCM (45–59 %), the activities of urease (5–17 %), invertase (7–28 %) and arylsulfatase (17–20 %) per soil mass, while increased C turnover rate (61–63 %), microbial metabolic quotient (qCO2) values (41–54 %) and enzyme activities per unit C and MBC (61–190 %) without affecting microbial quotient (MBC/C ratio) values and the activities of soil phosphatases per soil mass. The decreases in soil microbial and biochemical properties were largely the result of a substantial decline in soil C contents and aggregate stability by tillage activities in farmlands. Although the absolute activities of soil enzymes (i.e., activity per dry soil mass) showed inconsistent responses to land use changes and were even less responsive than soil organic C and MBC contents to changing land uses, the specific enzyme activities expressed either per unit of C or MBC tended to increase considerably and consistently with this conversion and were more sensitive than soil C and MBC contents. Soil C turnover rates and metabolic quotients were found to correlate positively with the specific enzyme activities, indicating an interaction between SOM and enzyme activities. The specific enzymatic activity could be an appropriate and integrative simple index to (1) assess soil quality changes, (2) detect changes in soil microbial community; and even (3) associate the losses of SOM with the presence of metabolically more active decomposer biota in rangelands cultivated for the long term.

Effects of pH change on microbial biomass-C and -P were examined using three red soils under citrus production but with different cultivation periods. Microbial biomass C and P were significantly affected by soil pH. The changes of Cmic and Pmic as a function of soil pH appeared to follow a normal distribution pattern with the original pH value as its center. Microbial biomass C or P was the greatest at the original soil pH and declined when the pH shifted to either acid or alkaline side. Moreover, there was a critical pH value at both the sides, beyond which the microbial biomass C or P abruptly decreased. The critical pH values were approximately 3.0 at the acid side and 8.0 to 8.5 at the alkaline side. The effect of pH on microbial biomass C and P was related to the original soil pH. The higher the original soil pH was, the less the microbial biomass C and P were affected by pH change. It is explained that soil microorganisms that grow in the soil environment with nearly neutral soil pH range (pH 5.5–7.5) may have greater tolerance to pH changes than those grow in the acid or alkaline soil pH conditions.

High specific activity in low microbial biomass soils across a no-till evapotranspiration gradient in Colorado

Microbial Biomass C, N and P in Disturbed Dry Tropical Forest Soils, India

Tillage-induced changes in fungal and bacterial biomass associated with soil aggregates: A long-term field study in a subtropical rice soil in China

14C-labelled glucose turnover in New Zealand soils

The effect of two types of organic amendment (manure and straw incorporation) and various doses (0–200 kg N*ha−1) of mineral N fertilization on microbial biomass C (MBC), aggregate stability (AS), soil organic C (SOC) and grain yield were investigated in an IOSDV long-term fertilization experiment (Keszthely, Hungary). This study was conducted during years 2015–2016 in a sandy loam Ramann-type brown forest soil (Eutric Cambisol according to WRB). Organic amendments had a significant effect on AS, MBC and SOC, increased their values compared to the unamended control. The organic amendments showed different effects on AS and MBC. AS was increased the most by straw incorporation and MBC by manure application. The magnitude of temporal variability of AS and MBC differed. Presumably, the different effects of organic amendments and the different degrees of temporal variability explain why there was only a weak (0.173) correlation between AS and MBC. AS did not correlate with SOC or grain yield. MBC correlated (0.339) with SOC but not with the grain yield. The N fertilizer dose did not have a significant effect on AS and MBC, but had a significant effect on SOC and grain yield.

The authors investigated the role of microorganisms in the paddy soil ecosystem and the influence of fertilizer and manure application on the soil “microbial biomass” in paddy soils using Jenkinson’s fumigation method. The results obtained are summarized as follows. 1) “Microbial biomass-C” was greatest in the organic manure plot, followed by the inorganic fertilizer plot, and smallest in the no fertilizer plot in each field. These results indicate that the different fertilizer managements remarkably influenced the “microbial biomass-C” measured by Jenkinson’s method. 2) The proportion of “microbial biomass-C” in total soil organic carbon was 3.6–6.9% in the no fertilizer plot, 4.3–7.5z% in the inorganic fertilizer plot, and 5.2–8.7% in the organic manure plot. These results show that fertilizer and manure application in paddy soils increased the proportion of “microbial biomass-C” in total soil organic carbon compared with that in grassland and forest soils. 3) There was no significant relationship between the “microbial biomass” determined by Jenkinson’s fumigation method and that by the direct counting method. “Microbial biomass-C” measured by Jenkinson’s method was usually larger than that by the direct microscopic method. 4) The relative coefficient between the soil “microbial biomass-C” and the mineralized nitrogen (NH4-N) under waterlogged conditions was 0.938 (n=l0). From these results, it appears that the fumigation with CHCl3 enhanced the decomposition of the non-microbial easily decomposable organic matter in paddy soil, and that the “microbial biomass” measured by Jenkinson’s fumigation method overestimates the real microbial biomass in paddy soils.

Nitrogen application can have a significant effect on soil carbon (C) pools, plant biomass production, and microbial biomass C processing. The focus of this study was to investigate the short-term effect of N fertilization on soil CO(2) emission and microbial biomass C. The study was conducted from 2001 to 2003 at four field sites in Iowa representing major soil associations and with a corn (Zea mays L.)-soybean (Glycine max L. Merr.) rotation. The experimental design was a randomized complete block with four replications of four N rates (0, 90, 180, and 225 kg ha(-1)). In the corn year, season-long cumulative soil CO(2) emission was greatest with the zero N application. There was no effect of N applied in the prior year on CO(2) emission in the soybean year, except at one of three sites, where greater applied N decreased CO(2) emission. Soil microbial biomass C (MBC) and net mineralization in soil collected during the corn year was not significantly increased with increase in N rate in two out of three sites. At all sites, soil CO(2) emission from aerobically incubated soil showed a more consistent declining trend with increase in N rate than found in the field. Nitrogen fertilization of corn reduced the soil CO(2) emission rate and seasonal cumulative loss in two out of three sites, and increased MBC at only one site with the highest N rate. Nitrogen application resulted in a reduction of both emission rate and season-long cumulative emission of CO(2)-C from soil.

Abundance and Dynamics of Soil Labile Carbon Pools Under Different Types of Forest Vegetation

An understanding of the microbial biomass dynamics in rice paddies is essential for managing their nutrient and C cycling. Our objectives were to determine whether the seasonal dynamics of microbial biomass C (MBC) was related to the release of organic substance from rice roots. MBC and the dissolved organic C (DOC) in soil solutions were measured over a growing period of rice plants in a pot experiment. The 13C pulse labelling (through supplying rice plants with 13CO2 for 6 h) was performed at different growth stages of rice to estimate the contribution of photosynthesized C to MBC. DOC concentrations increased with plant growth, reflecting the release of soluble root exudates from rice roots. MBC declined in the early period, and then rapidly increased from the maximum tillering stage to the heading stage of rice. During this period of increase, MBC was positively correlated to the DOC concentration and root biomass. About 0.15–0.94% (mean 0.54%) of photosynthesized 13C was incorporated into MBC immediately after pulse labelling, and 0.18–0.75% (mean 0.41%) still remained at the end of the season. The estimated total contribution of photosynthates to MBC amounted to 91 mg C plant–1, corresponding to 28% of total MBC at the end of the season or a 100% increase in MBC over the growing season. The results suggest that MBC dynamics in rice soil are largely controlled by organic substances released from rice roots.

Stoichiometry of soil microbial biomass carbon and microbial biomass nitrogen in China's temperate and alpine grasslands