Changes in Soil Organic Carbon and Its Labile Fractions after Land Conversion from Paddy Fields to Woodlands or Corn Fields

Land use change could significantly affect soil organic carbon (SOC) and other soil chemical properties. However, the responses of soil labile C fractions at different soil depths to land-use change are not still clear. The aim of this study was to investigate the effect of paddy field conversion on woodlands or corn fields on total soil organic C (TOC) and its labile C fractions including particulate organic C (POC), microbial biomass C (MBC), and potassium permanganate-oxidizable C (KMnO4–C) along a 0–100 cm soil profile. Our results indicate that soil TOC concentrations increased by 3.88 g kg−1 and 3.47 g kg−1 in the 0–5 cm soil layer and 5.33 g kg−1 and 4.68 g kg−1 in the 5–20 cm soil layer during 13 years after the conversion from paddy fields to woodlands and corn fields, respectively. In the 20–40 cm soil layer, the woodlands had the highest TOC concentration (12.3 g kg−1), which was 5.13 g kg−1 and 3.5 g kg−1 higher than that of the paddy and corn fields, respectively. The increase in TOC was probably due to the absence of soil disturbance and greater root residue input into the woodland soil. In corn fields, pig manure addition contributed to the increase in soil organic C concentrations. In addition, the proportion of soil KMnO4–C increased after conversion from paddy fields to woodlands or corn fields in the 0–40 cm soil layer, ranging from 39.9–56.6% for the woodlands and 24.6–32.9% for the corn fields. The soil POC content was significantly higher in woodland and corn field soils than in paddy field soils at lower soil depths (5–40 cm). However, there were no differences in MBC contents in the whole soil profile between the woodlands and paddy fields. The KMnO4–C and MBC was the most important factor affecting the CMI values through the whole 0–100 cm soil profile. Overall, converting paddy fields to woodlands or corn fields increased the TOC and labile C fractions in the 0–40 cm soil layer. Future studies should focus on the response of the deeper soil C pool to land-use change.

Warming and Labile Substrate Addition Alter Enzyme Activities and Composition of Soil Organic Carbon

Warming can increase the efflux of carbon dioxide (CO2) from soils and can potentially feedback to climate change. In addition to warming, the input of labile carbon can enhance the microbial activity by stimulating the co-metabolism of recalcitrant soil organic matter (SOM). This is particularly true with SOM under invaded ecosystems where elevated CO2 and warming may increase the biomass of invasive species resulting in higher addition of labile substrates. We hypothesized that the input of labile carbon would instigate a greater soil organic carbon (SOC) loss with warming compared to the ambient temperature. We investigated this by incubating soils collected from a native pine (Pinus taeda) forest to which labile carbon from the invasive species kudzu (Pueraria lobata) was added. We evaluated the microbial extracellular enzyme activity, molecular composition of SOC and the temperature sensitivity of soil CO2 efflux under warming and labile carbon addition. After 14 months of soil incubation, the addition of labile C through kudzu extract increased the activity of β-1,4-glucosidase compared with the control. However, the activity of N-acetyl-β-D-glucosaminidase and fungal biomass (ergosterol) decreased with labile carbon addition. The activity of peroxidase increased with warming after 14 months of soil incubation. Although the carbon content of incubated soils did not vary with substrate and temperature treatments, the molecular composition of SOC indicated a general decrease in biopolymers such as cutin, suberin, long-chain fatty acids, and phytosterol with warming and an increasing trend of microbial-derived compounds with labile substrate addition. In soils that received an addition of labile C, the macro-aggregate stability was higher while the temperature sensitivity of soil C efflux was lower compared with the control. The increase in aggregate stability could enhance the physical protection of SOC from microbial decomposition potentially contributing to the observed pattern of temperature sensitivity. Our results suggest that warming could preferentially accelerate the decomposition of recalcitrant compounds while the addition of labile substrates could enhance microbial-derived compounds that are relatively resistant to further decomposition. Our study further emphasizes that global change factors such as plant invasion and climate change can differentially alter soil microbial activity and the composition of SOC.

Biochar alleviates metal toxicity and improves microbial community functions in a soil co-contaminated with cadmium and lead

Soil amendment with biochar alleviates the toxic effects of heavy metals on microbial functions in single-metal contaminated soils. Yet, it is unclear how biochar application would improve microbial activity and enzymatic activity in soils co-polluted with toxic metals. The present research aimed at determining the response of microbial and biochemical attributes to addition of sugarcane bagasse biochar (SCB) in cadmium (Cd)-lead (Pb) co-contaminated soils. SCBs (400 and 600 °C) decreased the available concentrations of Cd and Pb, increased organic carbon (OC) and dissolved organic carbon (DOC) contents in soil. The decrease of metal availability was greater with 600 °C SCB than with 400 °C SCB, and metal immobilization was greater for Cd (16%) than for Pb (12%) in co-spiked soils amended with low-temperature SCB. Biochar application improved microbial activity and biomass, and enzymatic activity in the soils co-spiked with metals, but these positive impacts of SCB were less pronounced in the co-spiked soils than in the single-spiked soils. SCB decreased the adverse impacts of heavy metals on soil properties largely through the enhanced labile C for microbial assimilation and partly through the immobilization of metals. Redundancy analysis further confirmed that soil OC was overwhelmingly the dominant driver of changes in the properties and quality of contaminated soils amended with SCB. The promotion of soil microbial quality by the low-temperature SCB was greater than by high-temperature SCB, due to its higher labile C fraction. Our findings showed that SCB at lower temperatures could be applied to metal co-polluted soils to mitigate the combined effects of metal stresses on microbial and biochemical functions.

Nitrogen fertilization impact on soil carbon pools and their stratification and lability in subtropical wheat-mungbean-rice agroecosystems

Nitrogen (N) is the prime nutrient for crop production and carbon-based functions associated with soil quality. The objective of our study (2012 to 2019) was to evaluate the impact of variable rates of N fertilization on soil organic carbon (C) pools and their stocks, stratification, and lability in subtropical wheat (Triticum aestivum)—mungbean (Vigna radiata)—rice (Oryza sativa L) agroecosystems. The field experiment was conducted in a randomized complete block design (RCB) with N fertilization at 60, 80, 100, 120, and 140% of the recommended rates of wheat (100 kg/ha), mungbean (20 kg/ha), and rice (80 kg/ha), respectively. Composite soils were collected at 0–15 and 15–30 cm depths from each replicated plot and analyzed for microbial biomass (MBC), basal respiration (BR), total organic C (TOC), particulate organic C (POC), permanganate oxidizable C (POXC), carbon lability indices, and stratification. N fertilization (120 and 140%) significantly increased the POC at both depths; however, the effect was more pronounced in the surface layer. Moreover, N fertilization (at 120% and 140%) significantly increased the TOC and labile C pools when compared to the control (100%) and the lower rates (60 and 80%). N fertilization significantly increased MBC, C pool (CPI), lability (CLI), and management indices (CMI), indicating improved and efficient soil biological activities in such systems. The MBC and POC stocks were significantly higher with higher rates of N fertilization (120% and 140%) than the control. Likewise, higher rates of N fertilization significantly increased the stocks of labile C pools. Equally, the stratification values for POC, MBC, and POXC show evidence of improved soil quality because of optimum N fertilization (120–140%) to maintain and/or improve soil quality under rice-based systems in subtropical climates.

Soil profile connectivity can impact microbial substrate use, affecting how soil CO₂ effluxes are controlled by temperature

Determining controls on the temperature sensitivity of heterotrophic soil respiration remains critical to incorporating soil–climate feedbacks into climate models. Most information on soil respiratory responses to temperature comes from laboratory incubations of isolated soils and typically subsamples of individual horizons. Inconsistencies between field and laboratory results may be explained by microbial priming supported by cross-horizon exchange of labile C or N. Such exchange is feasible in intact soil profiles but is absent when soils are isolated from surrounding depths. Here we assess the role of soil horizon connectivity, by which we mean the degree to which horizons remain layered and associated with each other as they are in situ, on microbial C and N substrate use and its relationship to the temperature sensitivity of respiration. We accomplished this by exploring changes in C : N, soil organic matter composition (via C : N, amino acid composition and concentration, and nuclear magnetic resonance spectroscopy), and the δ13C of respiratory CO2 during incubations of organic horizons collected across boreal forests in different climate regions where soil C and N compositions differ. The experiments consisted of two treatments: soil incubated (1) with each organic horizon separately and (2) as a whole organic profile, permitting cross-horizon exchange of substrates during the incubation. The soils were incubated at 5 and 15 ∘C for over 430 d. Enhanced microbial use of labile C-rich, but not N-rich, substrates were responsible for enhanced, whole-horizon respiratory responses to temperature relative to individual soil horizons. This impact of a labile C priming mechanism was most emergent in soils from the warmer region, consistent with these soils' lower C bioreactivity relative to soils from the colder region. Specifically, cross-horizon exchange within whole soil profiles prompted increases in mineralization of carbohydrates and more 13C-enriched substrates and increased soil respiratory responses to warming relative to soil horizons incubated in isolation. These findings highlight that soil horizon connectivity can impact microbial substrate use in ways that affect how soil effluxes of CO2 are controlled by temperature. The degree to which this mechanism exerts itself in other soils remains unknown, but these results highlight the importance of understanding mechanisms that operate in intact soil profiles – only rarely studied – in regulating a key soil–climate feedback.

Effect of Peatland Siltation on Total and Labile C, N, P and K

Large areas of peatlands, in addition to the effect of drainage, were subjected to erosional process and were silted. The objective of the study was to verify whether siltation of peatlands hampers mineralization of remaining peat and alters labile C, N, P and K. Total C and N were measured on a CN analyzer, and total P and K on an ICP spectrometer after microwave digestion. The labile fractions of C, N, P and K were extracted with hot water and measured on the CN analyzer and ICP spectrometer. We noted that labile C, N, P and K concentrations in silted topsoil were lower than the values reported in unsilted topsoil. Higher concentration of labile compounds in peats is a signal of higher biological activity and mineralization of organic matter. A TOC/TP < 300 and TOC/TN of approximately 8 in topsoil suggested diminished mineralization and supported our hypothesis that siltation hampered mineralization of organic matter. The TOC/TK ratio proved to be a fine indicator of the state of organic soils siltation, which enabled the separation of unsilted peats from silted topsoil (on the base of value of 177). It can be assumed that the mineralization of peat layers is hampered by the above lying silted topsoil, which is less biologically active, having less oxygen, and therefore conserving underlying peats against oxidation.

Root exudates compounds and microbial community composition regulates CH4 dynamics in fire degraded tropical peatland

&lt;p&gt;Fires and drainage are common disturbance factors in tropical peatlands (TP) in Southeast Asia. These disturbances alter the hydrology, vegetation composition, and peat biogeochemistry; thereby affecting the microbiome where microbial communities reside&lt;strong&gt;. &lt;/strong&gt;Studies from northern peatlands have well established the role of vegetation composition in regulating the labile C, in the form of plant root exudates, and microbial community composition affecting the peat decomposition; however, for tropics, it remains unexplored. Recent studies have also established how these fire-degraded TP areas become a hot spot of sedge-mediated CH&lt;sub&gt;4&lt;/sub&gt; emission. To further our understanding of control mechanisms regulating CH&lt;sub&gt;4&lt;/sub&gt; dynamics, we investigated the composition of plant root exudates (n=3 per plant species) from sedges (Scleria sumatrensis) and ferns (Blechnum indicum, Nephrolepis hirsutula), the most commonly occurring plant species at our fire-degraded tropical peatland site in Brunei, Northwest Borneo, as well as microbial community composition in plant (n=9 for S. sumatrensis, and B. indicum, and n=5 for N. hirsutula) rhizo-compartments (rhizosphere, rhizoplane, endosphere).&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; Using a targeted analysis, we found that the root exudates compounds secreted from sedge (Scleria sumatrensis) and one species of fern (Blechnum indicum) were significantly different (p&lt;0.05) and showed a similar ratio of 2:1 for sugars (glucose, fructose) and organic acids (acetate, formate, lactate, malate, oxalate, succinate, tartrate), which is in contrast to that secreted from trees in intact tropical peatlands (1:2). Further, using 16S rRNA gene amplicon sequencing, we found that the microbial community composition in rhizo-compartments of plant species showed significant differences (p&lt;0.001). Interestingly, the sedge species harboured a relatively higher abundance of methanogens (Thermoplasmata) and lesser methanotrophs (Alphaproteobacteria, Gammaproteobacteria) across all three compartments compared to fern species, which further supports the higher sedge-mediated CH&lt;sub&gt;4&lt;/sub&gt; emissions from fire-degraded TP.&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; Our results provide fresh insights into the effects of post-fire vegetation composition in regulating the labile C and microbial community composition, and hence affecting CH&lt;sub&gt;4&lt;/sub&gt; emissions from fire-degraded TP. Further, our results can form an important basis for future CH&lt;sub&gt;4&lt;/sub&gt; dynamics studies as the emissions might increase with the expansion of degraded TPs as a consequence of frequent fire episodes and flooding&lt;/p&gt;

Effects of microtopography, root exudates analogues and temperature variation on CO2 and CH4 production from fire-degraded tropical peat

&lt;p&gt;Despite being an important terrestrial carbon (C) reserve, tropical peatlands (TP) have been heavily degraded through extensive drainage and fire, to an extent where degraded TP occupies one-tenth of the total peatland area in Southeast Asia (as in 2015). Consequently, repeated fires along with frequent flooding can alter the microtopography, vegetation composition as well as higher diurnal temperature variation due to open canopy, where each is known to influence C dynamics. However, assessing the importance of all these variables on-site can be challenging due to difficult site conditions; hence an incubation experiment approach may provide more useful insights in disentangling the complex interplay of these important variables in regulating GHG (CO&lt;sub&gt;2&lt;/sub&gt; and CH&lt;sub&gt;4&lt;/sub&gt;) production and emissions from fire-degraded tropical peatland areas. Therefore, we conducted an incubation study to investigate the interactions of microtopography (creating water-saturation conditions: mesic, flooded oxic, and anoxic), labile C inputs (in form of root exudate secretion from ferns and sedges), as well as on-site diurnal temperature variation in regulating CO&lt;sub&gt;2&lt;/sub&gt; and CH&lt;sub&gt;4&lt;/sub&gt; production from fire-degraded tropical peat.&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; We found that CO&lt;sub&gt;2&lt;/sub&gt; and CH&lt;sub&gt;4&lt;/sub&gt; production significantly varied among treatments and were strongly regulated by microtopography, labile C inputs, and temperature variation. Mesic (oxic) treatments acted as a strong source of CO&lt;sub&gt;2&lt;/sub&gt; (230.4 &amp;#177; 29 &amp;#181;gCO&lt;sub&gt;2 &lt;/sub&gt;g&lt;sup&gt;-1 &lt;/sup&gt;hr&lt;sup&gt;-1&lt;/sup&gt;) and mild sink for CH&lt;sub&gt;4&lt;/sub&gt; (-5.6 &amp;#177; 0.2 ngCH&lt;sub&gt;4 &lt;/sub&gt;g&lt;sup&gt;-1 &lt;/sup&gt;hr&lt;sup&gt;-1&lt;/sup&gt;) compared to anoxic treatments acting as a mild source of CO&lt;sub&gt;2&lt;/sub&gt; (61.3 &amp;#177; 6.2 &amp;#181;gCO&lt;sub&gt;2 &lt;/sub&gt;g&lt;sup&gt;-1 &lt;/sup&gt;hr&lt;sup&gt;-1&lt;/sup&gt;) and strong source of CH&lt;sub&gt;4 &lt;/sub&gt;(591.9 &amp;#177; 112.1 ngCH&lt;sub&gt;4 &lt;/sub&gt;g&lt;sup&gt;-1 &lt;/sup&gt;hr&lt;sup&gt;-1&lt;/sup&gt;). The addition of labile C enhanced both the CO&lt;sub&gt;2&lt;/sub&gt; and CH&lt;sub&gt;4&lt;/sub&gt; production irrespective of the treatment conditions, whereas the effect of diurnal temperature variation was clearly pronounced in mesic (for CO&lt;sub&gt;2&lt;/sub&gt;) and anoxic (for CH&lt;sub&gt;4&lt;/sub&gt;) conditions. Q&lt;sub&gt;10&lt;/sub&gt; values for both CO&lt;sub&gt;2&lt;/sub&gt; and CH&lt;sub&gt;4&lt;/sub&gt; production varied significantly with higher values for CO&lt;sub&gt;2&lt;/sub&gt; in mesic treatments (1.21 &amp;#177; 0.28) and higher for CH&lt;sub&gt;4&lt;/sub&gt; in anoxic treatments (1.56 &amp;#177; 0.35). We also observed a gradient across conditions, where flooded oxic treatments showed in-between values both for CO&lt;sub&gt;2&lt;/sub&gt; and CH&lt;sub&gt;4&lt;/sub&gt; production and temperature sensitivity, further reflecting the importance of on-site peat water-saturation in regulating the GHG production and emission from the fire degraded tropical peatland areas.&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; Overall, these findings highlight how the water-saturation conditions due to microtopographic variation in peat surface, quality, and quantity of labile C secreted from plant communities and temperature variation during a day can influence the GHGs production rates from the fire degraded tropical peat. More importantly, given the current state and extent of degraded tropical peatland areas and future climate and land-use changes as well as frequent fire episodes in the region, our results demonstrate the increasing trend in GHG production from the fire-degraded tropical peatlands in Southeast Asia.&lt;/p&gt;

Soil profile connectivity can impact microbial substrate use, affecting how soil CO₂ effluxes are controlled by temperature

Determining controls on the temperature sensitivity of heterotrophic soil respiration remains critical to incorporating soil-climate feedbacks into climate models. Most information on soil respiratory responses to temperature come from laboratory incubations of isolated soils, and typically subsamples of individual horizons. Inconsistencies between field and laboratory results may be explained by labile C or N priming supported by cross-horizon exchange – an indirect effect of quantifying microbial temperature response within intact soil profiles. Here we assess the role of soil horizon connectivity, by which we mean the degree to which horizons remain layered and associated with each another as they are in situ, on microbial C and N substrate use and its relationship to the temperature sensitivity of respiration. We accomplished this by exploring changes in C : N, soil organic matter composition (via amino acid composition and concentration, and nuclear magnetic resonance spectroscopy), and the δ13C of respiratory CO2 during incubations of organic horizons collected across boreal forests in different climate regions where soil C and N composition differ. The experiments consisted of two treatments: soil incubated (1) with each organic horizon separately, and (2) as a whole organic profile, permitting cross-horizon exchange of substrates during the incubation. The soils were incubated at 5 °C and 15 °C for over 430 days. Enhanced microbial use of labile C-rich, but not N-rich, substrates were responsible for enhanced, whole-horizon respiratory responses to temperature relative to individual soil horizons. This impact of a labile C priming mechanism was most emergent in soils from the warmer region, consistent with these soils' lower C bioreactivity relative to soils from the colder region. Specifically, cross-horizon exchange within whole soil profiles prompted increases in mineralization of carbohydrates and more 13C-enriched substrates and increased soil respiratory responses to warming relative to soil horizons incubated in isolation. These findings highlight that soil horizon connectivity can impact microbial substrate use in ways that affect how soil effluxes of CO2 are controlled by temperature. The degree to which this mechanism exerts itself in other soils remains unknown, but these results highlight the importance of understanding mechanisms that operate in intact soil profiles – only rarely studied – in regulating a key soil-climate feedback.