Effects of biochar, compost, and biochar-compost on soil total nitrogen and available phosphorus concentrations in a corn field in Papua New Guinea.

Replenishing soil nutrient particularly total nitrogen (TN) and available phosphorus (P) is important to sustain soil health for food production. Organic amendments such as compost and biochar are commonly applied to improve soil nutrient retention especially N and P. In farms, biochar is usually applied once followed by applying other organic amendment applied in their full rates. Both form and rate of organic amendments can affect soil nutrient concentrations particularly in short term. This study aimed to examine the effects of compost and mixture of compost with biochar (both at full rates) on soil nutrient concentrations in short term. A randomised complete block filed experiment with eight replicates was used for this study. The effects of biochar (5 t/ha) only, compost (at the rates of 10 t/ha, 25 t/ha and 35 t/ha) and biochar mixed with compost (5 t/ha and 10 t/ha, respectively) on soil nutrient concentrations compared with control were explored in a corn field. Compost treatment at the rate of 35 t/ha had significantly higher TN, available P, calcium (Ca) and iron (Fe) compared with other treatments and control. Soil potassium (K) levels remained unchanged among all treatments. Biochar only treatment had significantly higher available P and Ca concentrations compared with biochar mixed with compost treatment. Compost application at higher rate (35 t/ha) proved best practice to significantly increase TN and available P concentrations in short term. Significantly higher available P concentration in biochar only treatment compared with the biochar mixed with compost treatment could have been associated with stimulation of P immobilisation when biochar was mixed with compost. Our results indicated that the form and rate of organic amendments in short term cropping systems are important to be considered while applying to a volcanic soil to ensure N and P availability for plants are not compromised.

Temporal variations in litterfall biomass input and nutrient return under long-term prescribed burning in a wet sclerophyll forest, Queensland, Australia.

Litterfall helps maintaining nutrient return in forest ecosystems. However, the influence of long-term prescribed burning on the dynamics of litterfall biomass and carbon (C) and nitrogen (N) cycling is poorly understood. A 39-year old prescribed burning field trial in a wet sclerophyll forest, southeast Queensland, Australia, was used to investigate the interactive effects of prescribed fire regimes and temporal variation on the quantity and quality of litterfall and C and N return. Treatments included no burning (NB) since 1969, 2 yearly burning (2yrB; burned 19 times) and 4 yearly burning (4yrB; burned 9 times) since 1972. Litterfall was collected monthly on 32 occasions between 2011 and 2013. Significant temporal variation was observed in monthly and annual litterfall biomass. Both burning treatments had lower monthly inputs of total litterfall and leaf litter, mean annual cumulative litter biomass, litter C concentrations and C return via leaf litter, compared with the NB treatment. Most significant reductions in litter N concentrations and N return via litter were associated with 2yrB treatment. The 4yrB and the NB treatments did not differ significantly in terms of twig biomass, litterfall C:N ratios and N return via leaf litter. Despite both long-term prescribed burning treatments negatively impacting C return to the soil by reducing the quantity and quality of litter inputs, previous studies at the site suggest no difference in 0-10 cm soil organic carbon levels between the 4yrB treatment and the unburnt treatment. Hence a longer period of prescribed burning at the 4yrB frequency is likely required before lower C return translates to differences in ecosystem productivity in this wet sclerophyll forest ecosystem. The 2yrB can potentially alter forest C and N cycling and net primary productivity, but these alterations are unlikely to be detected through short-term studies.

Responses of labile soil organic carbon and nitrogen pools to long-term prescribed burning regimes in a wet sclerophyll forest of southeast Queensland, Australia.

Soil labile organic carbon (C) and nitrogen (N) pools play a central role in nutrient cycling, while fire is a key driver of biogeochemical cycle, shaping ecosystem structure and functioning. However, how soil labile organic C and N responds to the long-term repeated prescribed fire is largely unknown. In this study, a prescribed fire field experiment in a wet sclerophyll forest established in 1972 in southeast Queensland was used to evaluate the long-term impacts of different fire frequency regimes on labile organic C and N measured by different extraction methods. The fire frequency regimes included long unburnt (NB), burnt every two years (2yrB) and burnt every four years (4yrB). Results revealed that the 2yrB treatment had significantly lower C and N concentrations in hot water and K2SO4 extracts and in density fractions (LFD<2.3 and HFD>1.6) compared with the NB treatment. Concentrations of carbohydrate-C in hot water extracts and acid soluble and insoluble organic matter-C in cold-water extracts followed a similar trend. The maximum reduction was observed for carbohydrate­C (72%) and the hot water extractable N (54%) in the 2yrB treatment compared with the NB treatment, showing these parameters are most sensitive indicators. However, there was no significant difference in most of the above parameters between the 4yrB and the NB treatments, indicating that less frequent fire (4yrB) allows the ecosystem to have sufficient time to recover from fire disturbance and may be a sustainable practice for fire management in this wet sclerophyll forest ecosystem.

Long-term frequent prescribed fire decreases surface soil carbon and nitrogen pools in a wet sclerophyll forest of Southeast Queensland, Australia.

Prescribed fire is one of the most widely-used management tools for reducing fuel loads in managed forests. However the long-term effects of repeated prescribed fires on soil carbon (C) and nitrogen (N) pools are poorly understood. This study aimed to investigate how different fire frequency regimes influence C and N pools in the surface soils (0-10 cm). A prescribed fire field experiment in a wet sclerophyll forest established in 1972 in southeast Queensland was used in this study. The fire frequency regimes included long unburnt (NB), burnt every 2 years (2yrB) and burnt every 4 years (4yrB), with four replications. Compared with the NB treatment, the 2yrB treatment lowered soil total C by 44%, total N by 54%, HCl hydrolysable C and N by 48% and 59%, KMnO4 oxidizable C by 81%, microbial biomass C and N by 42% and 33%, cumulative CO2-C by 28%, NaOCl-non-oxidizable C and N by 41% and 51%, and charcoal-C by 17%, respectively. The 4yrB and NB treatments showed no significant differences for these soil C and N pools. All soil labile, biologically active and recalcitrant and total C and N pools were correlated positively with each other and with soil moisture content, but negatively correlated with soil pH. The C:N ratios of different C and N pools were greater in the burned treatments than in the NB treatments. This study has highlighted that the prescribed burning at four year interval is a more sustainable management practice for this subtropical forest ecosystem.