Nitrogen turnover and N2O/N2 ratio of three contrasting tropical soils amended with biochar.

Biochar has been reported to reduce emission of nitrous oxide (N2O) from soils, but the mechanisms responsible remain fragmentary. For example, it is unclear how biochar effects on N2O emissions are mediated through biochar effects on soil gross N turnover rates. Hence, we conducted an incubation study with three contrasting agricultural soils from Kenya (an Acrisol cultivated for 10-years (Acrisol10); an Acrisol cultivated for over 100-years (Acrisol100); a Ferralsol cultivated for over 100 years (Ferralsol)). The soils were amended with biochar at either 2% or 4% w/w. The 15N pool dilution technique was used to quantify gross N mineralization and nitrification and microbial consumption of extractable N over a 20-day incubation period at 25 °C and 70% water holding capacity of the soil, accompanied by N2O emissions measurements. Direct measurements of N2 emissions were conducted using the helium gas flow soil core method. N2O emissions varied across soils with higher emissions in Acrisols than in Ferralsols. Addition of 2% biochar reduced N2O emissions in all soils by 53 to 78% with no significant further reduction induced by addition at 4%. Biochar effects on soil nitrate concentrations were highly variable across soils, ranging from a reduction, no effect and an increase. Biochar addition stimulated gross N mineralization in Acrisol-10 and Acrisol-100 soils at both addition rates with no effect observed for the Ferralsol. In contrast, gross nitrification was stimulated in only one soil but only at a 4% application rate. Also, biochar effects on increased NH4 + immobilization and NO3 -consumption strongly varied across the three investigated soils. The variable and bidirectional biochar effects on gross N turnover in conjunction with the unambiguous and consistent reduction of N2O emissions suggested that the inhibiting effect of biochar on soil N2O emission seemed to be decoupled from gross microbial N turnover processes. With biochar application, N2 emissions were about an order of magnitude higher for Acrisol-10 soils compared to Acrisol-100 and Ferralsol-100 soils. Our N2O and N2 flux data thus support an explanation of direct promotion of gross N2O reduction by biochar rather than effects on soil extractable N dynamics. Effects of biochar on soil extractable N and gross N turnover, however, might be highly variable across different soils as found here for three typical agricultural soils of Kenya.

Biome-specific effects of nitrogen and phosphorus on the photosynthetic characteristics of trees at a forest-savanna boundary in Cameroon.

Photosynthesis/nutrient relationships of proximally growing forest and savanna trees were determined in an ecotonal region of Cameroon (Africa). Although area-based foliar N concentrations were typically lower for savanna trees, there was no difference in photosynthetic rates between the two vegetation formation types. Opposite to N, area-based P concentrations were-on average-slightly lower for forest trees; a dependency of photosynthetic characteristics on foliar P was only evident for savanna trees. Thus savanna trees use N more efficiently than their forest counterparts, but only in the presence of relatively high foliar P. Along with some other recent studies, these results suggest that both N and P are important modulators of woody tropical plant photosynthetic capacities, influencing photosynthetic metabolism in different ways that are also biome specific. Attempts to find simple unifying equations to describe woody tropical vegetation photosynthesis-nutrient relationships are likely to meet with failure, with ecophysiological distinctions between forest and savanna requiring acknowledgement.

Quantifying the abundance and stable isotope composition of pyrogenic carbon using hydrogen pyrolysis.

RATIONALE: Pyrogenic carbon (C(P)) is an important component of the global carbon budget. Accurate determination of the abundance and stable isotope composition of C(P) in soils and sediments is crucial for understanding the dynamics of the C(P) cycle and interpreting records of biomass burning, climate and vegetation change in the past. Here we test hydrogen pyrolysis (hypy) as a new technique potentially capable of eliminating labile organic carbon (C(L)) from total organic carbon (C(T)) in a range of matrices in order to enable reliable quantification of both the C(P) component of C(T) and the stable carbon isotope composition of C(P) (δ(13)C(P)). METHODS: We mixed C(P) at a range of concentrations with common C(P)-free matrices (C(L) = cellulose, chitin, keratin, decomposed wood, leaf litter, grass and algae) and determined the amount of residual carbon not removed by hydrogen pyrolysis (C(R)) as a ratio of C(T) (C(R)/C(T)). Mixing C(P) with a unique δ(13)C value provided a natural abundance isotope label from which to precisely determine the ratio of C(P) to residual C(L) remaining after hypy. RESULTS: All C(P)-free matrices contained trace carbon after hypy, indicating that hypy does not remove all the C(L). However, there was a strong correlation between C(R)/C(T) and C(P)/C(T), viz. C(R)/C(T)= 1.02(C(P)/C(T)) + 4.0 × 10(-3), r(2) = 0.99, p <0.001, suggesting that only a small and reasonably constant fraction of C(L) remains after hypy. Uncertainties associated with the correction for contamination of C(R) by residual C(L) are minimal allowing for reliable determinations of both C(P) and δ(13)C(P) in many cases. CONCLUSIONS: Hydrogen pyrolysis appears to be a robust technique for estimating C(P) abundance and δ(13)C(P) across a range of materials. Nevertheless, caution is required in interpreting δ(13)C(P) values when C(P)/C(T) is low, with C(P)/C(T)>4% being required for the determination of the δ(13)C(P) values within an interpretable error under our experimental conditions.

Co-limitation of photosynthetic capacity by nitrogen and phosphorus in West Africa woodlands.

Photosynthetic leaf traits were determined for savanna and forest ecosystems in West Africa, spanning a large range in precipitation. Standardized major axis fits revealed important differences between our data and reported global relationships. Especially for sites in the drier areas, plants showed higher photosynthetic rates for a given N or P when compared with relationships from the global data set. The best multiple regression for the pooled data set estimated V(cmax) and J(max) from N(DW) and S. However, the best regression for different vegetation types varied, suggesting that the scaling of photosynthesis with leaf traits changed with vegetation types. A new model is presented representing independent constraints by N and P on photosynthesis, which can be evaluated with or without interactions with S. It assumes that limitation of photosynthesis will result from the least abundant nutrient, thereby being less sensitive to the allocation of the non-limiting nutrient to non-photosynthetic pools. The model predicts an optimum proportionality for N and P, which is distinct for V(cmax) and J(max) and inversely proportional to S. Initial tests showed the model to predict V(cmax) and J(max) successfully for other tropical forests characterized by a range of different foliar N and P concentrations.

Linkages between soil carbon, soil fertility and nitrogen fixation in Acacia senegal plantations of varying age in Sudan.

BACKGROUND: Over the last decades sub-Saharan Africa has experienced severe land degradation and food security challenges linked to loss of soil fertility and soil organic matter (SOM), recurrent drought and increasing population. Although primary production in drylands is strictly limited by water availability, nutrient deficiencies, particularly of nitrogen (N) and phosphorus (P), are also considered limiting factors for plant growth. It is known that SOM (often measured as soil organic carbon (SOC)) is a key indicator of soil fertility, therefore, management practices that increase SOM contents, such as increasing tree cover, can be expected to improve soil fertility. The objectives of this study were to investigate the effect of Acacia senegal (Senegalia senegal) trees on soil nitrogen, phosphorus and potassium (K) in relation to SOC, the potential of A. senegal for N2 fixation, and to identify possible N and P ecosystem limitations. METHODS: Soil nutrient (total N, P, K and available P and exchangeable K) concentrations and stocks were determined for the 0-10, 10-20,20-30 and 30-50 cm layers of A. senegal plantations of varying age (ranging from 7 to 24-years-old) and adjacent grasslands (reference) at two sites in semi-arid areas of Sudan. At both sites, three plots were established in each grassland and plantation. The potential of A. senegal for N2 fixation in relation to plantations age was assessed using δ15N isotopic abundances and nutrient limitations assessed using C:N:P stoichiometry. RESULTS: Soil concentrations of all studied nutrients were relatively low but were significantly and directly correlated to SOC concentrations. SOC and nutrient concentrations were the highest in the topsoil (0-10 cm) and increased with plantations age. Acacia foliage δ15N values were >6‰ and varied little with plantations age. Soil C:N and C:P ratios did not differ between grassland and plantations and only 0-10 cm layer N:P ratios showed significant differences between grassland and plantations. DISCUSSION: The results indicated that soil fertility in the Sahel region is strongly related to SOM contents and therefore highlighting the importance of trees in the landscape. The higher mineral nutrient concentrations in the topsoil of the plantations may be an indication of 'nutrient uplift' by the deeper roots. The high foliar δ15N values indicated that N2 fixation was not an important contributor to soil N contents in the plantations. The accretion of soil N cannot be explained by deposition but may be related to inputs of excreted N brought into the area annually by grazing and browsing animals. The soil C:N:P stoichiometry indicated that the plantations may be limited by P and the grasslands limited by N.

Foliar trait contrasts between African forest and savanna trees: genetic versus environmental effects.

Variations in leaf mass per unit area (Ma) and foliar concentrations of N, P, C, K, Mg and Ca were determined for 365 trees growing in 23 plots along a West African precipitation gradient ranging from 0.29 to 1.62m a-1. Contrary to previous studies, no marked increase in Ma with declining precipitation was observed, but savanna tree foliar [N] tended to be higher at the drier sites (mass basis). Generally, Ma was slightly higher and [N] slightly lower for forest vs savanna trees with most of this difference attributable to differences in soil chemistry. No systematic variations in [P], [Mg] and [Ca] with precipitation or between trees of forest vs savanna stands were observed. We did, however, find a marked increase in foliar [K] of savanna trees as precipitation declined, with savanna trees also having a significantly lower [K] than those of nearby forest. These differences were not related to differences in soil nutrient status and were accompanied by systematic changes in [C] of opposite sign. We suggest an important but as yet unidentified role for K in the adaption of savanna species to periods of limited water availability; with foliar [K] being also an important factor differentiating tree species adapted to forest vs savanna soils within the 'zone of transition' of Western Africa.