Nutrient storage in soil and biomass of native Brazilian Cerrado

The removal or burning of the biomass which frequently includes main roots results in significant nutrient losses from the Brazilian savanna, the Cerrado. To estimate these losses, we quantified above‐ and belowground plant biomass and total nutrient storage in biomass and soil of a typical Cerrado. Dominant tree species in the layer > 2 m were Pouteria torta (MART. ) RADLK ., Ouratea spectabilis (MART .) ENGL ., Roupala montana AUBL ., Byrsonima coccolobifolia H.B. et K., Dalbergia miscolobium BENTH ., Kielmeyera coriacea MART ., and Caryocar brasiliense CAMBESS . which together represented 70 % of the biomass of the > 2 m layer. In the 0.5—2 m tree layer, many different species were found of which Ouratea hexasperma (ST .‐HIL .) BAILL . representing 33 % of the biomass in the 0.5—2 m layer was most abundant. The dominant shrub species were Miconia holosericea DC., Hortia brasiliana VAND . ex DC., Myrcia rostrata DC., Parinari obtusifolia HOOK . f., and Campomanesia velutina BLUME , contributing 93 % to the total shrub biomass. Total aboveground plant biomass was 22.7 Mg ha^—1, total belowground plant biomass was 30.4 Mg ha^—1. The tree layer > 2 m comprised the largest proportion of the aboveground biomass (64.6 %) > grass/herb (13.0 %) > shrub layer (11.6 %) > tree layer 0.5—2 m (10.8 %). Three quarters of the fine root biomass (17.6 Mg ha^—1) were located in the upper 0.3 m of the soil. The element storages (in kg ha^—1) were C: 10900, N: 173 N, P: 20, K: 51, Ca: 66, Mg: 20, S: 25, Fe: 10, Mn: 4.2, Zn: 0.35, and Al: 27 in the aboveground biomass, C: 12900, N: 214 N, P: 14, K: 41, Ca: 52, Mg: 10, S: 33, Fe: 2060, Mn: 2.9, Zn: 0.60, and Al: 648 in the belowground biomass, and C: 55400, N: 3510 N, P: 631, K: 366, Ca: 86, Mg: 75, S: 529, Fe: 159000, Mn: 124, Zn: 49, and Al: 434000 in the soil (0—0.3 m). If the above‐ and belowground biomass was completely removed from the Cerrado ecosystem losses would range from 5 % of the total nutrient storage for P to 58 % for Ca referred to a lower ecosystem boundary at 0.3 m mineral soil depth.     

Effects of fertilizer application and fire regime on soil microbial biomass carbon and nitrogen,and nitrogen mineralization in an Australian subalpine eucalypt forest

The effects of a range of fertilizer applications and of repeated low-intensity prescribed fires on microbial biomass C and N, and in situ N mineralization were studied in an acid soil under subalpine Eucalyptus pauciflora forest near Canberra, Australia. Fertilizer treatments (N, P, N+P, line + P, sucrose + P), and P in particular, tended to lower biomass N. The fertilizer effects were greatest in spring and smaller in summer and late actumn. Low-intensity prescribed fire lowered biomass N at a soil depth of 0–5 cm with the effect being greater in the most frequently burnt soils. No interactions between fire treatments, season, and depth were significant. Only the lime + P and N+P treatments significantly affected soil microbial biomass C contents. The N+P treatment increased biomass C only at 0–2.5 cm in depth, but the soil depth of entire 0–10 cm had much higher (>doubled) biomass C values in the line + P treatment. Frequent (two or three times a year) burning reduced microbial boomass C, but the reverse was true in soils under forest burn at intervals of 7 years. Soil N mineralization was increased by the addition of N and P (alone or in combination), line + P, and sucrose + P to the soil. The same was true for the ratio of N mineralization to biomass N. Soil N mineralization was retarded by repeated fire treatments, especially the more frequent fire treatment where rates were only about half those measured in unburnt soils. There was no relationship between microbial biomass N (kg N ha^-1) and the field rates of soil N mineralization (kg N ha^-1 month^-1). The results suggest that although soil microbial biomass N represents a distinct pool of N, it is not a useful measure of N turnover.     

Biochars influence sweet‐potato yield and nutrient uptake in tropical Papua New Guinea

Currently, the biomass of an invasive and obnoxious weed, kunai grass (Imperata cylindrica), is uncontrollably burnt in Papua New Guinea in subsistence farming systems resulting in unwarranted negative environmental consequences. We explored the possibility of sustainable utilization of biochar produced from the weed biomass along with a standard feedstock‐rice husk (Oryza sativa). Biochars were produced with lab‐scale pyrolysis at 550°C, characterized for chemical properties and plant nutrient composition. Further, agronomic efficacy of soil incorporation of biochars (5 t ha^?1) or co‐applied with mineral fertilizers (100, 11, and 62 kg ha^?1 N, P, K, respectively) was tested for sweet potato (Ipomoea batatas L. Lam) in a field experiment. The two biochars differed significantly (P < 5%) with respect to recovery from the feedstocks, chemical characters and nutrient composition. Kunai grass biochar was poorer in nutrients (< 1%) with distinctly alkaline pH and higher electrical conductivity. Biochar amendment to soil showed significant (P < 5%) improvement of soil moisture, while co‐application of biochars along with mineral fertilizers showed soil moisture decrease. Biochar amendment improved the growth parameters and total tuber yield of sweet potato by about 20%, while co‐application with mineral fertilizers augmented total tuber yield by 100% and above‐ground biomass yields by > 75%. Besides, improving agronomic performance of sweet potato crop, co‐application of biochars with mineral fertilizers enhanced uptake of N, P, K, Ca, Mg, and S. Production and utilization of biochar in sweet‐potato production could offer an efficient means of disposing biomass of kunai grass with concomitant productivity improvement in Papua New Guinea.     

Above- and belowground ecosystem biomass and carbon pools in an age-sequence of temperate pine plantation forests

We assessed the successional development of above- and belowground ecosystem biomass and carbon (C) pools in an age-sequence of four White pine (Pinus strobus L.) plantation stands (2-, 15-, 30-, and 65-years-old) in Southern Ontario, Canada. Biomass and C stocks of above- and belowground live and dead tree biomass, understorey and forest ground vegetation, forest floor C (LFH-layer), and woody debris were determined from plot-level inventories and destructive tree sampling. Small root biomass (<5 mm) and mineral soil C stocks were estimated from soil cores. Aboveground tree biomass became the major ecosystem C pool with increasing age, reaching 0.5, 66, 92, and 176 t ha⁻¹ in the 2-, 15-, 30-, and 65-year-old stands, respectively. Tree root biomass increased from 0.1 to 10, 18, 38 t ha⁻¹ in the 2-, 15-, 30-, and 65-year-old stands, respectively, contributing considerably to the total ecosystem C in the three older stands. Forest floor C was 0.8, 7.5, 5.4, and 12.1 t C ha⁻¹ in the 2-, 15-, 30-, and 65-year-old stands, respectively, indicating an increase during the first two decades, but no further age-effect during the later growth phase. Mineral soil C was age-independent with 37.2, 33.9, 39.1, and 36.7 t C ha⁻¹ in the 2-, 15-, 30-, and 65-year-old stands, respectively. Aboveground ecosystem C increased with age from 3 to 40, 52, and 100 t C ha⁻¹ in the 2-, 15-, 30-, and 65-year-old stands, respectively, due to an increase in aboveground tree biomass. Belowground ecosystem C remained similiar in the early decades after establishment with 37, 39, and 39 t C ha⁻¹ in the 2-, 15-, and 30-year-old stands, but increased to 56 t C ha⁻¹ in the 65-year-old stand due to an increase in root biomass. The difference in total ecosystem C between the 2- and 65-year-old stand was 116 t C ha⁻¹. Our results highlight the importance of considering the successional development of forest ecosystem C pools, when estimating C sink potentials over their complete life cycle.     

The Warrumbungle Post‐Fire Recovery Project—raising the profile of soils

The impacts of a wildfire and subsequent rainfall event in 2013 in the Warrumbungle National Park in New South Wales, Australia were examined in a project designed to provide information on post‐fire recovery expectations and options to land managers. A coherent suite of sub‐projects was implemented, including soil mapping, and studies on soil organic carbon (SOC) and nitrogen (N), erosion rates, groundcover recovery and stream responses. It was found that the loss of SOC and N increased with fire severity, with the greatest losses from severely burnt sandstone ridges. Approximately 2.4 million t of SOC and ~74,000 t of N were lost from soil to a depth of 10 cm across the 56,290 ha affected. Soil loss from slopes during the subsequent rainfall event was modelled up to 25 t ha^?1, compared to a long‐term mean annual soil loss of 1.06 t ha^?1 year^?1. Groundcover averages generally increased after the fire until spring 2015, by which time rates of soil loss returned to near pre‐fire levels. Streams were filled with sand to bank full levels after the fire and rainfall. Rainfall events in 2015–2016 shifted creek systems into a major erosive phase, with incision through the post‐fire sandy bedload deposits, an erosive phase likely related to loss of topsoils over much of the catchment. The effectiveness of the research was secured by a close engagement with park managers in issue identification and a communications programme. Management outcomes flowing from the research included installation of erosion control works, redesign of access and monitoring of key mass movement hazard areas.     

Gradients in N‐cycling attributes along forestry and agricultural land‐use systems are indicative of soil capacity for N supply

Indicators of soil quality associated with N‐cycling were assessed under different land‐use systems (native forest – NAT, reforestation with Araucaria angustifolia or Pinus taeda and agricultural use – AGR) to appraise the effects on the soil potential for N supply. The soil total N ranged from 2 to 4 g/kg (AGR and NAT, respectively), and the microbial biomass N ranged from 80 to 250 mg/kg, being higher in NAT and A. angustifolia, and lower in P. taeda and AGR sites. Activities of asparaginase (ca. 50–200 mg NH₄⁺‐N/kg per h), glutaminase (ca. 200–800 mg NH₄⁺‐N/kg per h) and urease (ca. 80–200 mg NH₄⁺‐N/kg/h) were also more intense in the NAT and A. angustifolia‐reforested soils, indicating greater capacity for N mineralization. The NAT and AGR soils showed the highest and the lowest ammonification rate, respectively (ca. 1 and 0.4 mg NH₄⁺‐N/kg per day), but the inverse for nitrification rate (ca. 12 and 26%), indicating a low capacity for N supply, in addition to higher risks of N losses in the AGR soil. A multivariate analysis indicated more similarity between NAT and A. angustifolia‐reforested sites, whilst the AGR soil was different and associated with a higher nitrification rate. In general, reforestation with the native species A. angustifolia had less impact than reforestation with the exogenous species P. taeda, considering the soil capacity for N supply. However, AGR use caused more changes, generally decrease in indicators of N‐cycling, showing a negative soil management effect on the sustainability of this agroecosystem.     

Carbon sequestration and relationship between carbon addition and storage under rainfed soybean–wheat rotation in a sandy loam soil of the Indian Himalayas

Soil organic matter (SOM) contributes to the productivity and physical properties of soils. Although crop productivity is sustained mainly through the application of organic manure in the Indian Himalayas, no information is available on the effects of long-term manure addition along with mineral fertilizers on C sequestration and the contribution of total C input towards soil organic C (SOC) storage. We analyzed results of a long-term experiment, initiated in 1973 on a sandy loam soil under rainfed conditions to determine the influence of different combinations of NPK fertilizer and fertilizer + farmyard manure (FYM) at 10 Mg ha⁻¹ on SOC content and its changes in the 0–45 cm soil depth. Concentration of SOC increased 40 and 70% in the NPK + FYM-treated plots as compared to NPK (43.1 Mg C ha⁻¹) and unfertilized control plots (35.5 Mg C ha⁻¹), respectively. Average annual contribution of C input from soybean (Glycine max (L.) Merr.) was 29% and that from wheat (Triticum aestivum L. Emend. Flori and Paol) was 24% of the harvestable above-ground biomass yield. Annual gross C input and annual rate of total SOC enrichment were 4852 and 900 kg C ha⁻¹, respectively, for the plots under NPK + FYM. It was estimated that 19% of the gross C input contributed towards the increase in SOC content. C loss from native SOM during 30 years averaged 61 kg C ha⁻¹ yr⁻¹. The estimated quantity of biomass C required to maintain equilibrium SOM content was 321 kg ha⁻¹ yr⁻¹. The total annual C input by the soybean–wheat rotation in the plots under unfertilized control was 890 kg ha⁻¹ yr⁻¹. Thus, increase in SOC concentration under long-term (30 years) rainfed soybean–wheat cropping was due to the fact that annual C input by the system was higher than the required amount to maintaining equilibrium SOM content.     

Analysis of the 1:5 soil: water extract in burnt soils to evaluate fire severity

This paper studies fire severity through changes in the composition of the 1:5 soil: water extract in burnt and unburnt samples collected after a wildfire that affected 7.1 ha of mountainous broom scrub (Teide broom, Spartocytisus supranubius L.) in Tenerife (Canary Islands). Soil samples were collected over a regular sampling grid and analysed for pH, electrical conductivity (E.C.), and soluble cations (Ca⁺², Mg⁺², K⁺, and NH₄⁺–N) and anions (P, SO₄^− 2–S, and NO₃⁻?N). A visual estimation of fire effects on individual broom plants was also conducted. The results show significant differences between burnt and unburnt samples for pH, E.C. and soluble ions (except for P), which generally agrees with data reported in the literature. Although fire severity, as estimated from the degree of fuel combustion, correlated significantly with most of the studied parameters, the qualitative fire severity indexes failed to discriminate sample groups apparently exposed to moderate fire severity conditions. Multivariate statistical analysis (PCA) allowed extracting two main factors, capable to account for 88% of total variability, after rejecting three variables (inorganic N-forms and K⁺). These factors were related to fire severity and pH-dependent Ca⁺² and P reactions, respectively.     

Tropical savannah woodland: effects of experimental fire on soil microorganisms and soil emissions of carbon dioxide

Burning of the vegetation in the African savannahs in the dry season is widespread and may have significant effects on soil chemical and biological properties. A field experiment in a full factorial randomised block design with fire, ash and extra grass biomass as main factors was carried out in savannah woodland of the Gambella region in Ethiopia. The microbial biomass C (C_mic) was 52% (fumigation-extraction) and 20% (substrate-induced respiration) higher in burned than unburned plots 12 d after burning. Both basal respiration and potential denitrification enzyme activity (PDA) immediately responded to burning and increased after treatment. However, in burned plots addition of extra biomass (fuel load) led to a reduction of C_mic and PDA due to enhanced fire temperature. Five days after burning, there was a short-lived burst in the in situ soil respiration following rainfall, with twice as high soil respiration in burned than unburned plots. In contrast, 12 d after burning soil respiration was 21% lower in the burned plots, coinciding with lower soil water content in the same plots. The fire treatment resulted in higher concentrations of dissolved organic C (24-85%) and nitrate (47-76%) in the soil until 90 d after burning, while soil NH₄⁺-N was not affected to the same extent. The increase in soil NO₃⁻-N but not NH₄⁺-N in the burned plots together with the well-aerated soil conditions indicated that nitrifying bacteria were stimulated by fire and immediately oxidised NH₄⁺-N to NO₃⁻-N. In the subsequent rainy season, NO₃⁻-N and, consequently, PDA were reduced by ash deposition. Further, C_mic was lower in burned plots at that time. However, the fire-induced changes in microbial biomass and activity were relatively small compared to the substantial seasonal variation, suggesting transient effects of the low severity experimental fire on soil microbial functioning.     

Seasonal effect of the exotic invasive plant Solidago gigantea on soil pH and P fractions

Invasions by alien plants can alter biogeochemical cycles in recipient ecosystems. We test if Early Goldenrod (Solidago gigantea) alters P fractions. To that end, we compare invaded plots and adjacent, uninvaded resident vegetation for specific fractions of organic and inorganic P, phosphomonoesterase (PME) activity in topsoil, and immobilization of P in above‐ and belowground organs and in soil microbial biomass. Invaded plots had lower soil pH and 20%–30% higher labile P fractions (resin‐P_i, bicarb‐P_i, NaOH‐Pi), and the difference was consistent across seasons. There was no difference in microbial P. Alkaline‐PME activity was 30% lower in topsoil of invaded plots. Annual P uptake in aboveground phytomass was not markedly higher in Solidago. In contrast, P in belowground organs steadily increased in autumn in invaded plots, due to both increased biomass and increased P concentrations. This indicated higher net P immobilization in Solidago, far in excess of both resorption from senescing shoots and P requirements for aboveground biomass in subsequent year. Higher turnover rates of P in belowground organs and mobilization of sparingly soluble P forms through rhizosphere acidification may be involved in the observed differences in soil P status between invaded and uninvaded plots.     

Effects of nitrogen enrichment on belowground communities in grassland: Relative role of soil nitrogen availability vs. soil acidification

Terrestrial ecosystems worldwide are receiving increasing amounts of biologically reactive nitrogen (N) as a consequence of anthropogenic activities. This intended or unintended fertilization can have a wide range of impacts on the above- and belowground communities. An increase in high N availability has been assumed to be a major mechanism enhancing the abundance of above- and belowground communities. In addition to increasing available N, however, N enrichment causes soil acidification, which may negatively affect above- and belowground communities. The relative importance of increased N availability vs. increased soil acidity for above- and belowground communities in natural ecosystems experiencing N enrichment is unclear. In a 12-year N enrichment experiment in a semi-arid grassland, N enrichment substantially increased both above- and belowground plant biomass mainly via the N availability-induced increase in biomass of perennial rhizome grasses. N enrichment also dramatically suppressed bacterial, fungal, and actinobacteria biomass mainly via the soil acidification pathway (acidification increased concentrations of H⁺ ions and Al³⁺ and decreased concentrations of mineral cations). In addition, N enrichment also suppressed bacterial-, fungal-feeding, and omnivorous + carnivorous nematodes mainly via the soil acidification pathway (acidification reduced nematode food resources and reduced concentrations of mineral cations). The positive effects resulting from the increase in belowground carbon allocation (via increase in quantity and quality of plant production) on belowground communities were outweighed by the negative effects resulting from soil acidification, indicating that N enrichment weakens the linkages between aboveground and belowground components of grassland ecosystems. Our results suggest that N enrichment-induced soil acidification should be included in models that predict biota communities and linkages to carbon and nitrogen cycling in terrestrial ecosystems under future scenarios of N deposition.     

CO2 evolution and N mineralization after biogas slurry application in the field and its yield effects on spring barley

The objective of this study was to investigate the effects of biogas slurry derived from straw-rich farmyard manure on the soil microbial biomass, on the mineralization in the field and on the related crop yield. The experiment was carried out in the following four treatments: (1) fallow, (2) fallow + biogas slurry, (3) spring barley, and (4) spring barley + biogas slurry. The CO₂ evolution rate ranged between 15 and 120 mg C m⁻² h⁻¹ in both fallow treatments and showed a significant exponential relationship with the soil temperature at 5 cm depth. According to the extrapolation of the CO₂ evolution rates into amounts per hectare, approximately 200 kg C ha⁻¹ or 27% of the biogas slurry derived C were mineralized to CO₂ during a 50 days’ period to 18 June in the fallow treatment with biogas slurry. An additional amount of up to 29.5 kg inorganic N ha⁻¹ could be calculated as the sum of NH₄-N already present in biogas slurry at the time of amendment and from the amount of biogas slurry mineralized in the soil to NO₃-N. A good agreement between measured and modelled stocks of inorganic N at 0–60 cm depth was obtained after having five-fold increased soil organic C turnover compared to the default values of the model DNDC. The mineralization data are in line with an amount of up to 21 kg ha⁻¹ more N transferred by the barley plants to their aboveground biomass in biogas slurry treatment. The N not accounted for by the aboveground plant biomass could be explained by the belowground plant-derived N. CO₂ evolution from the soil surface, inorganic N content at 0–60 cm depth and N transfer into barley aboveground biomass lead apparently to similar results after the application of biogas slurry. The soil ATP content after harvest of the barley was significantly larger in the two treatments with biogas slurry, especially in the fallow treatment indicating a positive effect on the soil microbial community.     

Effects of compost type and storage conditions on climbing bean on Technosols of Tantalum mining sites in Western Rwanda

Little is known about the effects of compost application to reclaim artisanal mining sites for agriculture in Central Africa. A field experiment was therefore conducted to examine the effects of locally available organic household waste composted under traditional (pit under leaf shade) versus improved management (pit under double plastic sheeting) and mixed with either Tithonia diversifolia biomass or Minjingu Phosphate Rock (13–15% P) on climbing bean sown on degraded Technosols (former Tantalum mining sites) and un‐mined control soils (Cambisols). Both soil types were derived from pegmatite. After 6 months of composting, nutrient concentrations in traditional compost were 0.27–0.32% N, 0.06–0.08% P, and 0.20–0.22% K. Comparative values in amended compost were 1.02–1.65% N, 0.10–0.31% P, and 0.41–1.13% K. In farmyard+solid waste, composted under traditional system, dry matter was 65.4%, pH 6.7, and C : N ratio 13.0, as opposed to 81.5% DM, a pH of 8.6, and a C : N ratio of 8.6 in farmyard+solid waste+Minjingu phosphate under improved compost, and 68.3% dry matter, a pH of 8.4, and a C : N ratio of 7.4 for Tithonia +farmyard+solid waste under improved conditions. Compared to bean (Phaseolus vulgaris L.) grain yields of 0.28 (mined soil) and 0.11 (unmined soil) without amendments, the application (on a dry matter basis) of 5 t compost ha⁻¹ led to yields of 3.54 t DM ha⁻¹ for improved compost Tithonia +farmyard+solid waste on mined soil versus 2.26 t DM ha⁻¹ (P < 5%) for the same treatment at the un‐mined sites. The yield obtained for farmyard+solid waste+Minjingu phosphate composted under improved conditions averaged 3.06 t DM ha⁻¹ at mined sites compared with 2.85 t DM ha⁻¹ at un‐mined sites (P > 5%). All amendments were more effective in enhancing bean yields on Technosols with significant positive effects with improved compost than on Cambisols.     

The use of biologically based phosphorus fractions to evaluate soil P availability in reduced P‐input paddy soils

Chemical soil phosphorus (P) extraction has been widely used to characterize and understand changes in soil P fractions; however, it does not adequately capture rhizosphere processes. In this study, we used the biologically based phosphorus (BBP ) grading method to evaluate the availability and influencing factors of soil P under four P fertilizer regimes in a typical rice–wheat cropping rotation paddy field. Soil P was assessed after seven rice‐growth seasons at multiple growth stages: the seedling, the booting and the harvest stage. Soil CaCl₂‐P, citrate‐P and HC l‐P (inorganic P, Pi) as well as enzyme‐P (organic P, Po) were not significantly different between soil treated with P fertilizer during the wheat season only (PW ) and during the rice season only (PR ) compared with soil treated during both the rice and the wheat seasons (PR +W) at all three rice‐growth stages. No P fertilizer application during either season (Pzero) significantly reduced the concentration of soil citrate‐P and HC l‐P at the rice‐seedling and harvest stages. Significant correlations were observed between the HC l extraction and Olsen‐P (R ² = 0.823, P  <  0.001), followed by enzyme‐P (R ² = 0.712, P  <  0.001), citrate‐P (R ² = 0.591, P  <  0.001) and CaCl₂‐P (R ² = 0.133, P  <  0.05). Further redundancy analysis (RDA ) suggested that soil alkaline phosphatase (S‐ALP ) activity played a role in soil P speciation changes and was significantly correlated with enzyme‐P, citrate‐P and HC l‐P. These results may improve our ability to characterize and understand changes in soil P status while minimizing the overapplication of P fertilizer.     

Enhancing phosphorus availability,soil organic carbon,maize productivity and farm profitability through biochar and organic–inorganic fertilizers in an irrigated maize agroecosystem under semi‐arid climate

Biochar amendments offer promising potential to improve soil fertility, soil organic carbon (SOC) and crop yields; however, a limited research has explored these benefits of biochar in the arid and semi‐arid regions. This two‐year field study investigated the effects of Acacia tree biomass‐derived biochar, applied at 0 and 10 t ha^?1 rates with farmyard manure (FYM) or poultry manure (PM) and mineral phosphorus (P) fertilizer combinations (100 kg P ha^‐1), on maize (Zea mays L.) productivity, P use efficiency (PUE) and farm profitability. The application of biochar with organic–inorganic P fertilizers significantly increased soil P and SOC contents than the sole organic or inorganic P fertilizers. Addition of biochar and PM as 100% P source resulted in the highest soil P (104% increase over control) and SOC contents (203% higher than control). However, maize productivity and PUE were significantly higher under balanced P fertilizer (50% organic + 50% mineral fertilizer) with biochar and the increase was 110%, 94% and 170% than 100%‐FYM, 100%‐PM and 100% mineral fertilizer, respectively. Maize productivity and yield correlated significantly positively with soil P and SOC contents These positive effects were possibly due to the ability of biochar to improve soil properties, P availability from organic–inorganic fertilizers and SOC which resulted in higher PUE and maize productivity. Despite the significant positive relationship of PUE with net economic returns, biochar incorporation with PM and mineral fertilizer combination was economically profitable, whereas FYM along biochar was not profitable due to short duration of the field experiments.     

Soil carbon dioxide flux, carbon sequestration and crop productivity in a tropical dryland agroecosystem: Influence of organic inputs of varying resource quality

In view of the significance of agricultural soils in affecting global C balance, the impact of manipulation of the quality of exogenous inputs on soil CO₂–C flux was studied in rice–barley annual rotation tropical dryland agroecosystem. Chemical fertilizer, Sesbania shoot (high quality resources), wheat straw (low quality resource) and Sesbania + wheat straw (high + low quality), all carrying equivalent recommended dose of N, were added to soil. A distinct seasonal variation in CO₂–C flux was recorded in all treatments, flux being higher during rice period, and much reduced during barley and summer fallow periods. During rice period the mean CO₂–C flux was greater in wheat straw (161% increase over control) and Sesbania + wheat straw (+129%) treatments; however, during barley and summer fallow periods differences among treatments were small. CO₂–C flux was more influenced by seasonal variations in water-filled pore space compared to soil temperature. In contrast, the role of microbial biomass and live crop roots in regulating soil CO₂–C flux was highly limited. Wheat straw input showed smaller microbial biomass with a tendency of rapid turnover rate resulting in highest cumulative CO₂–C flux. The Sesbania input exhibited larger microbial biomass with slower turnover rate, leading to lower cumulative CO₂–C flux. Addition of Sesbania to wheat straw showed higher cumulative CO₂–C flux yet supported highest microbial biomass with lowest turnover rate indicating stabilization of microbial biomass. Although single application of wheat straw or Sesbania showed comparable net change in soil C (18% and 15% relative to control, respectively) and crop productivity (32% and 38%), yet they differed significantly in soil C balance (374 and −3 g C m⁻² y⁻¹ respectively), a response influenced by the recalcitrant and labile nature of the inputs. Combining the two inputs resulted in significant increment in net change in soil C (33% over control) and crop yield (49%) in addition to high C balance (152 g C m⁻² y⁻¹). It is suggested that appropriate mixing of high and low quality inputs may contribute to improved crop productivity and soil fertility in terms of soil C sequestration.     

Nitrous oxide emission from a transplanted rice field in alluvial soil as influenced by management of nitrogen fertiliser

We investigated nitrous oxide (N₂O) emission from an irrigated rice field over two years to evaluate the management of nitrogenous fertiliser and its effect on reducing emissions. Four forms of nitrogenous fertilisers: NPK at the recommended application rate, starch–urea matrix (SUM) + PK, neem‐coated urea + PK and urea alone (urea without coating) were used. Gas samples were collected from the field at weekly intervals with the static chamber technique. N₂O emissions from different treatments ranged from 11.58 to 215.81 N₂O‐N μg/m²/h, and seasonal N₂O emissions from 2.83 to 3.89 kg N₂O‐N/ha. Compared with other fertilisers, N₂O emissions were greatest after the application of the conventional NPK fertiliser. Moreover, SUM + PK reduced total N₂O emissions by 22.33% (P < 0.05) compared with NPK during the rice‐growing period (P < 0.05). The results indicate a strong correlation between N₂O emissions and soil organic carbon, nitrate, ammonium, above‐ and below‐ground plant biomass and photosynthesis (P < 0.05). The application of SUM + PK in rice fields is suitable as a means of reducing N₂O emissions without affecting grain production.     

Short-term effect of wildfire on the chemical, biochemical and microbiological properties of Mediterranean pine forest soils

The short-term effects of wildfire on the characteristics of Mediterranean pine forest soils, exposed to semiarid climatic conditions, were evaluated by measuring different chemical, biochemical and microbiological parameters 9 months after the fire. Soils in which the fire had been intense showed higher electrical conductivity values than unburnt soils. All burnt soils had higher contents of nitrates, exchangeable NH₄ ⁺ and available P and K while their contents of total organic C, extractable C, humic acids, water-soluble C and total and water-soluble carbohydrates were, in general, lower than those of unburnt soils. Microbial biomass-C in burnt soils represented from 50% to 79% of that of unburnt soils; basal respiration and dehydrogenase activity were also negatively affected by fire. In general, fire decreased urease and N-α-benzoyl-l-argininamide hydrolysing protease activities. Alkaline phosphatase activity in burnt soils was 29–87% that of the respective unburnt control soil. Arylsulphatase activity was also lower in burnt soils as was β-glucosidase activity, although in this case the differences from values of unburnt soils were not always statistically significant. Received: 15 July 1996     

Short-term effects of prescribed burning on phosphorus availability in a suburban native forest of subtropical Australia

Purpose

Prescribed burning can alter nutrient availability to plants. Plant growth in tropical and subtropical forests is frequently phosphorus (P) limited. Soil P availability is influenced by a combination of multiple factors including soil chemical and biological properties. The aims of this study were to investigate the short-term effects of prescribed burning on soil P status and to evaluate the key drivers responsible for the variation in soil P fractions.

Materials and methods

Soil samples were collected at a depth of 0–10 cm at two sites in a suburban native forest. One site (the burnt site) was burned on 11 August 2011. The other site (the control site) was not burned but served as a reference. Sampling was conducted at four times: before burning, 12 days after burning (T1), 1 week after T1 (T2), and 1 month after T2 (T3). Soil pH, P fractions, microbial biomass carbon (C) and P, and activities of acid and alkaline phosphatase were measured.

Results and discussion

Total P was relatively low at both sites compared with other subtropical forests. Microbial biomass P accounted for approximately 10 % of soil total P at the two sites, suggesting that the turnover of microbial biomass is critical for soil P availability. Soil properties at the control site remained unchanged over the time. Soil organic forms of P at the burnt site were decreased by the prescribed burning, and the greatest reduction was found in moderately labile organic P (e.g., NaOH-extractable fractions). Soil inorganic forms of P, however, were not correspondingly increased by the prescribed burning. Microbial biomass P was closely related to the shifts in P fractions. These effects were only detected immediately after the fire.

Conclusions

Microbial biomass could serve as a sink of P in P-impoverished soils and play an important role in soil P transformation. Our results indicate that microbial biomass is an important factor that governs P status after prescribed burning. The rapid recovery of microbial biomass P could be beneficial to the P requirement for plant regrowth after prescribed burning.     

Soil loss and run‐off characteristics under different soil amendments and cropping systems in the semi‐deciduous forest zone of Ghana

Soil erosion is a major constraint to crop production on smallholder arable lands in Sub‐Saharan Africa (SSA). Although different agronomic and mechanical measures have been proposed to minimize soil loss in the region and elsewhere, soil management practices involving biochar‐inorganic inputs interactions under common cropping systems within the framework of climate‐smart agriculture, have been little studied. This study aimed to assess the effect of different soil and crop management practices on soil loss characteristics under selected cropping systems, typical of the sub‐region. A two‐factor field experiment was conducted on run‐off plots under different soil amendments over three consecutive cropping seasons in the semi‐deciduous forest zone of Ghana. The treatments, consisting of three soil amendments (inorganic fertilizer, biochar, inorganic fertilizer + biochar and control) and four cropping systems (maize, soyabean, cowpea, maize intercropped with soyabean) constituted the sub‐plot and main plot factors, respectively. A bare plot was included as a soil erosion check. Seasonal soil loss was greater on the bare plots, which ranged from 9.75–14.5 Mg ha^?1. For individual crops grown alone, soil loss was 31%–40% less under cowpea than under maize. The soil management options, in addition to their direct role in plant nutrition, contributed to significant (p < 0.05) reductions in soil loss. The least soil loss (1.23–2.66 Mg ha^?1) was observed under NPK fertilizer + biochar treatment (NPK + BC) over the three consecutive cropping seasons. Biochar in combination with NPK fertilizer improved soil moisture content under cowpea crops and produced considerably smaller bulk density values than most other treatments. The NPK + BC consistently outperformed the separate mineral fertilizer and biochar treatments in biomass yield under all cropping systems. Biochar associated with inorganic fertilizers gave economic returns with value–cost ratio (VCR) > 2 under soyabean cropping system but had VCR < 2 under all other cropping systems. The study showed that biochar/NPK interactions could be exploited in minimizing soil loss from arable lands in SSA.