Effect of moisture on leaf litter decomposition and its contribution to soil respiration in a temperate forest

The degree to which increased soil respiration rates following wetting is caused by plant (autotrophic) versus microbial (heterotrophic) processes, is still largely uninvestigated. Incubation studies suggest microbial processes play a role but it remains unclear whether there is a stimulation of the microbial population as a whole or an increase in the importance of specific substrates that become available with wetting of the soil. We took advantage of an ongoing manipulation of leaf litter <sup>14</sup>C contents at the Oak Ridge Reservation, Oak Ridge, Tennessee, to (1) determine the degree to which an increase in soil respiration rates that accompanied wetting of litter and soil, following a short period of drought, could be explained by heterotrophic contributions; and (2) investigate the potential causes of increased heterotrophic respiration in incubated litter and 0–5 cm mineral soil. The contribution of leaf litter decomposition increased from 6 ± 3 mg C m<sup>−2</sup> hr<sup>−1</sup> during a transient drought, to 63 ± 18 mg C m<sup>−2</sup> hr<sup>−1</sup> immediately after water addition, corresponding to an increase in the contribution to soil respiration from 5 ± 2% to 37 ± 8%. The increased relative contribution was sufficient to explain all of the observed increase in soil respiration for this one wetting event in the late growing season. Temperature (13°C versus 25°C) and moisture (dry versus field capacity) conditions did not change the relative contributions of different decomposition substrates in incubations, suggesting that more slowly cycling C has at least the same sensitivity to decomposition as faster cycling organic C at the temperature and moisture conditions studied

Response of Soil Respiration to Soil Temperature and Moisture in a 50-Year-Old Oriental Arborvitae Plantation in China

China possesses large areas of plantation forests which take up great quantities of carbon. However, studies on soil respiration in these plantation forests are rather scarce and their soil carbon flux remains an uncertainty. In this study, we used an automatic chamber system to measure soil surface flux of a 50-year-old mature plantation of Platycladus orientalis at Jiufeng Mountain, Beijing, China. Mean daily soil respiration rates (Rs) ranged from 0.09 to 4.87 µmol CO2 m−2s−1, with the highest values observed in August and the lowest in the winter months. A logistic model gave the best fit to the relationship between hourly Rs and soil temperature (Ts), explaining 82% of the variation in Rs over the annual cycle. The annual total of soil respiration estimated from the logistic model was 645±5 g C m−2 year−1. The performance of the logistic model was poorest during periods of high soil temperature or low soil volumetric water content (VWC), which limits the model's ability to predict the seasonal dynamics of Rs. The logistic model will potentially overestimate Rs at high Ts and low VWC. Seasonally, Rs increased significantly and linearly with increasing VWC in May and July, in which VWC was low. In the months from August to November, inclusive, in which VWC was not limiting, Rs showed a positively exponential relationship with Ts. The seasonal sensitivity of soil respiration to Ts (Q10) ranged from 0.76 in May to 4.38 in October. It was suggested that soil temperature was the main determinant of soil respiration when soil water was not limiting

Allocation, stress tolerance and carbon transport in plants: How does phloem physiology affect plant ecology?

Despite the crucial role of carbon transport in whole plant physiology and its impact on plant-environment interactions and ecosystem function, relatively little research has tried to examine how phloem physiology impacts plant ecology. In this review, we highlight several areas of active research where inquiry into phloem physiology has increased our understanding of whole plant function and ecological processes. We consider how xylem-phloem interactions impact plant drought tolerance and reproduction, how phloem transport influences carbon allocation in trees and carbon cycling in ecosystems, and how phloem function mediates plant relations with insects, pests, microbes and symbiotes. We argue that in spite of challenges that exist in studying phloem physiology, it is critical that we consider the role of this dynamic vascular system when examining the relationship between plants and their biotic and abiotic environment

Characterisation and evaluation of biochars for their application as a soil amendment

Biochar properties can be significantly influenced by feedstock source and pyrolysis conditions; this warrants detailed characterisation of biochars for their application to improve soil fertility and sequester carbon. We characterised 11 biochars, made from 5 feedstocks ['Eucalyptus saligna' wood (at 400°C and 550°C both with and without steam activation); 'E. saligna' leaves (at 400°C and 550°C with activation); papermill sludge (at 550°C with activation); poultry litter and cow manure (each at 400°C without activation and at 550°C with activation)] using standard or modified soil chemical procedures. Biochar pH values varied from near neutral to highly alkaline. In general, wood biochars had higher total C, lower ash content, lower total N, P, K, S, Ca, Mg, Al, Na, and Cu contents, and lower potential cation exchange capacity (CEC) and exchangeable cations than the manure-based biochars, and the leaf biochars were generally in-between. Papermill sludge biochar had the highest total and exchangeable Ca, CaCO₃ equivalence, total Cu, and potential CEC, and the lowest total and exchangeable K. Water-soluble salts were higher in the manure-based biochars, followed by leaf, papermill sludge, and wood biochars. Total As, Cd, Pb, and polycyclic aromatic hydrocarbons in the biochars were either very low or below detection limits. In general, increase in pyrolysis temperature increased the ash content, pH, and surface basicity and decreased surface acidity. The activation treatment had a little effect on most of the biochar properties. X-ray diffraction analysis showed the presence of whewellite in 'E. saligna' biochars produced at 400°C, and the whewellite was converted to calcite in biochars formed at 550°C. Papermill sludge biochar contained the largest amount of calcite. Water-soluble salts and calcite interfered with surface charge measurements and should be removed before the surface charge measurements of biochar. The biochars used in the study ranged from C-rich to nutrient-rich to lime-rich soil amendment, and these properties could be optimised through feedstock formulation and pyrolysis temperature for tailored soil application

Impact of Management on Soil Carbon and Nutrient Cycling and Storage Under Contrasting Farming Systems

Soil organic matter (SOM) is a key indicator of soil quality, regulating major soil processes and functions such as soil organic carbon (SOC) storage and cycling, microbial biomass and activity, and nutrient storage and cycling in agro-ecosystems. There has been increasing interest in these functions of SOM, and how they are impacted by management practices in different farming systems. However, there is limited understanding of how, and to what extent, soil structural units (termed "aggregate-size classes") with different SOM bioavailability, influenced by contrasting tillage practices, mediate these soil processes. Moreover, knowledge on the allocation dynamics of newly assimilated C and N in crop–soil systems, with implications for nutrient use efficiency and crop productivity, is also limited. An improved understanding of these inter-relationships will provide insights into identifying land management practices with potential to increase SOM storage, while enhancing plant available nutrients, nutrient use efficiency and therefore crop productivity at farm scale. A series of experiments was carried out to enhance understanding of the impact of different management practices on key soil processes and functions such as: soil structural stability; carbon and nutrient cycling, availability and storage; microbial biomass and activity; and the coupling between plant C input and soil nutrient availability or N uptake by plants. These processes were examined in bulk soil and/or in different aggregate-size classes. The experiments reported in Chapters 2-5 were performed using soils from three long-term (16-46 years) sites, i.e. the Condobolin (NSW) and Merredin (WA) sites, on a Luvisol, with a semi-arid or a Mediterranean climate, respectively, and the Hermitage (QLD) site, on a Vertisol, with a sub-tropical climate. The practices at Condobolin comprised conventional (CT) and reduced tillage (RT) with mixed crop-pasture rotation, no-till (NT) with continuous cropping, and perennial pasture (PP. The practices at Merredin comprised stubble either retained (SR) or burnt (SB) under direct-drilled continuous cropping. The practices at Hermitage comprised a factorial combination of CT, NT, SR, SB, with either 0 (0N) or 90 kg urea-N ha-1 (90N) in a continuous cropping system. For the study in Chapter 2, dry and wet sieving techniques were used to separate mega- (> 2 mm), macro- (2-0.25 mm), micro-aggregate (0.25-0.053 mm) and silt-plus-clay ( 2 mm), macro- (2-0.25 mm), micro-aggregate (0.25-0.053 mm) and silt-plus-clay (i.e. 0-10 cm, 10-20 cm and 20-30 cm depths). To understand the processes of SOC mineralisation and the release of plant available nutrients, as impacted by different tillage management practices, the surface layer (0-10 cm) bulk soil and soil aggregates [mega-aggregate, macro-aggregate and micro-aggregate (Chapter 3-5). The first study (Chapter 2) demonstrated that management practices with relatively low or no soil disturbance improved soil structure in the Luvisol at Condobolin, while the high clay content in the Vertisol at Hermitage may have overridden the effect of tillage, and therefore, management had minimal impact on soil structure stability. Further, this study found minor to modest impacts of long-term management practices on soil C gain and N, S and P stocks across the field sites. The so-called improved management practices such as perennial pasture at Condobolin, and no-till, stubble retention and fertilisation at Hermitage showed relatively higher SOC and nutrient stocks than management practices with disturbance through tillage plus stubble burning. The findings suggest that the least disturbed systems along with stubble retention and fertilisation can enhance agricultural sustainability by increasing SOC and nutrient concentrations, particularly in micro-aggregates or micro-structures. The second study, (Chapter 3), showed that SOM has a significant fertiliser value in terms of the supply of plant available N, P and S, and management practices can significantly influence the release of plant available nutrients from SOM. In this experiment, bulk soils (Luvisol and Vertisol) collected from 14 long-term management practices across the three long-term field sites were incubated for 126 days. The mineralisation of SOC and the release of nutrients were higher in the CT versus RT and NT, and the SR versus SB practices in both soils. This study found a continuous release of plant available N across all the management practices over the study period, whereas, the release of available P and S was evident only during the first 30 days, after which P and S availability decreased, probably because microbial immobilisation or clay fixation of P and S predominated, particularly in the Vertisol. These findings suggest that SOM is a direct source of nutrients for crop growth, and management practices involving soil disturbance along with organic matter (residues) input can promote SOM mineralisation and the release of plant available nutrients in farming systems. The third study, another incubation study (Chapter 4), suggested that SOM can continuously release plant available nutrients (particularly P and S) over 126 days after incorporation of residues. In this experiment, crop residues [canola (Brassica napus: δ13C 124‰) or wheat (Triticum aestivum: δ13C 461‰) stem] were added to Luvisol (δ13C -24.7‰) and Vertisol (δ13C -18.5‰) sampled from the contrasting tillage (CT or RT and NT) treatments and incubated for 126 days. This study found that crop residue input into the tilled systems stimulated native SOC mineralisation by ~100-300% across both soils. Both SOC mineralisation and the release of plant available nutrients varied with tillage intensity (CT or RT > NT), residue type (canola > wheat), and soil type (Vertisol > Luvisol). This study also found that crop residue input ( NT), residue type (canola > wheat), and soil type (Vertisol > Luvisol). This study also found that crop residue input ( wheat), and soil type (Vertisol > Luvisol). This study also found that crop residue input ( Luvisol). This study also found that crop residue input (cf. control) did not change the magnitude of net available N over the study period, possibly due to stronger N immobilisation than mineralisation. However, a significant amount of available P and S was released in both soils over 126 days. Therefore, this study suggests that, in addition to the likely release of available P and S from the residues via mineralisation, considerable quantities of available P and S may have been released from the soil reserves via positive priming of SOM mineralisation (as demonstrated in our study) and possibly via dissolution/desorption reactions in the soils. Results from the fourth study (Chapter 5) demonstrated that the differently sized aggregate classes had a smaller effect, compared with the effects of tillage intensity, residue type and soil clay content/type, on the priming of native SOC mineralisation and nutrient (N, P and S) release dynamics in the soils. In this laboratory incubation study, the 13C-labelled canola or wheat stem residues were added into the three dry aggregate-size classes, collected from contrasting tillage systems on the Luvisol and Vertisol, and incubated for 126 days. This study found that crop residue input (versus no residue input) stimulated SOC mineralisation in all three aggregatesize classes in both soils. The native SOC mineralisation varied with tillage intensity (CT > RT > NT) (in the Luvisol only), residue type (canola > wheat), and aggregate-size classes (macro- ≥ micro- > mega-aggregates) in both soils. Crop residue input into the soil aggregates ( RT > NT) (in the Luvisol only), residue type (canola > wheat), and aggregate-size classes (macro- ≥ micro- > mega-aggregates) in both soils. Crop residue input into the soil aggregates ( NT) (in the Luvisol only), residue type (canola > wheat), and aggregate-size classes (macro- ≥ micro- > mega-aggregates) in both soils. Crop residue input into the soil aggregates ( wheat), and aggregate-size classes (macro- ≥ micro- > mega-aggregates) in both soils. Crop residue input into the soil aggregates ( mega-aggregates) in both soils. Crop residue input into the soil aggregates (cf. no residue input) maintained the release of available N over the study period, likely due to the dominance of microbial N immobilisation versus mineralisation induced by the relatively C-rich and nutrient-poor crop residues. An interesting finding is that incorporation of crop residues released a considerable amount of plant available nutrients (particularly P and S) from the soil aggregate reserves, most likely via different biological (e.g. priming) and chemical (e.g. nutrient desorption and mineral dissolution) mechanisms. Further, micro- and/or macro- versus mega-aggregates, canola versus wheat residue input, and Vertisol versus Luvisol had higher plant available nutrients in till versus no-till systems. To understand the allocation dynamics of newly assimilated C and N in a canola crop-soil system with different tillage and N fertilisation treatments, a field-based 13C15N isotopic study was performed at Wagga Wagga, NSW (Chapter 6). Results from this study showed that short-term tillage and N fertilisation can increase belowground allocation of newly assimilated C and plant uptake of soil-released N, under a semi-arid environment. In this study, in situ 13CO2 and urea-15N pulse labelling was conducted during the canola flowering stage. Despite no short-term effect of management practices on total SOC and N stocks, aggregate stability, microbial biomass, and 13C retention in different aggregate-size classes, this study found greater new root C input to 1 m depth, plant N uptake and canola seed yield under a low-intensity tillage (cf.. no-till) and N fertilisation (cf. no N) system. To conclude, the studies showed that tillage along with stubble retention and/or N fertilisation can stimulate SOM decomposition and the release of plant available nutrients, while enhancing plant derived-C input and nutrient uptake, with positive implications for crop productivity in farming systems. On the contrary, the least disturbed systems such as perennial pasture or no-till along with stubble retention and N fertilisation can increase SOC and nutrient concentrations in micro-structures, although these systems (cf.. tilled systems) gave modest gains in SOC and nutrient stocks. Using tillage to encourage nutrient release should be planned strategically, as the timing of nutrient release from the stimulated SOM decomposition needs to be aligned with crop demand to maximise efficiency of use of nutrients. A further consideration is the trade-off with climate change mitigation, as encouraging SOM mineralisation via tillage will decrease SOC stocks, particularly in surface soil layers

Influence of biochars on nitrous oxide emission and nitrogen leaching from two contrasting soils

The influence of biochar on nitrogen (N) transformation processes in soil is not fully understood. This study assessed the influence of four biochars (wood and poultry manure biochars synthesized at 400°C, nonactivated, and at 550°C, activated, abbreviated as: W400, PM400, W550, PM550, respectively) on nitrous oxide (N₂O) emission and N leaching from an Alfisol and a Vertisol. Repacked soil columns were subjected to three wetting–drying (W–D) cycles to achieve a range of water-filled pore space (WFPS) over a 5-mo period. During the first two W–D cycles, W400 and W550 had inconsistent effects on N₂O emissions and the soils amended with PM400 produced higher N₂O emissions relative to the control. The initially greater N₂O emission from the PM400 soils was ascribed to its higher labile intrinsic N content than the other biochars. During the third W–D cycle, all biochar treatments consistently decreased N₂O emissions, cumulatively by 14 to 73% from the Alfisol and by 23 to 52% from the Vertisol, relative to their controls. In the first leaching event, higher nitrate leaching occurred from the PM400-amended soils compared with the other treatments. In the second event, the leaching of ammonium was reduced by 55 to 93% from the W550- and PM550-Alfisol and Vertisol, and by 87 to 94% from the W400- and PM400-Vertisol only (cf. control). We propose that the increased effectiveness of biochars in reducing N₂O emissions and ammonium leaching over time was due to increased sorption capacity of biochars through oxidative reactions on the biochar surfaces with ageing