Malate exudation by aerobic rice (Oryza sativa L.) grown on a low Zn soil

It has been frequently suggested that root exudates play a role in Zn mobilization. We studied root exudation of low-molecular weight organic anions (LMWOAs) in three aerobic rice genotypes in relation to their Zn uptake efficiency. Plants were grown in a rhizotron filled with a Zn deficient clay soil added with or without 5 mg Zn kg-1 soil as ZnSO4. Soil solution was sampled when symptoms of Zn deficiency for –Zn plants appeared. We measured LWMOAs in the soil solution and found that plants responded to Zn deficiency with increased (64%) root exudation of malate. The efficient genotype exuded higher amounts of malate than the inefficient genotype. The genotype with medium Zn uptake efficiency, however, exuded less malate than the inefficient genotype. No significant correlation between malate concentrations in rhizosphere and plant Zn uptake efficiency was found. These results suggest that Zn mobilization by soil-grown rice genotypes cannot be explained by increased malate exudation and the variation in plant Zn uptake is most likely not be explained by their capacity to exude LMWOAs as a response to Zn deficiency

Nitrogen losses from two grassland soils with different fungal biomass.

Nitrogen losses from agricultural grasslands cause eutrophication of ground- and surface water and contribute to global warming and atmospheric pollution. It is widely assumed that soils with a higher fungal biomass have lower N losses, but this relationship has never been experimentally confirmed. With the increased attention for soil-based ecosystem services and sustainable management of soils, such a relationship would be relevant for agricultural management. Here we present a first attempt to test this relationship experimentally . We used intact soil columns from two plots from a field experiment that had consistent differences in fungal biomass (68 ± 8 vs. 111 ± 9 μg C g-1) as a result of different fertilizer history (80 vs. 40 kg N ha-1 y-1 as farm yard manure), while other soil properties were very similar. In the greenhouse, the columns received either mineral fertilizer N or no N (control). We measured N leaching, N2O emissions and denitrification from the columns during 4 weeks, after which we analyzed fungal and bacterial biomass and soil N pools. We found that N2O emission and denitrification were lower in the high fungal biomass soil, irrespective of the addition of fertilizer N. After fertilizer addition, N leaching in low fungal biomass soil showed a 3-fold increase compared to the control (11.9 ± 1.0 and 3.9 ± 1.0 kg N ha-1, respectively), but did not increase in high fungal biomass soil (6.4 ± 0.9 after N addition vs. 4.5 ± 0.8 kg N ha-1 in the control). Thus, in the high fungal biomass soil more N was immobilized. An additional experiment with 15N–labelled mineral fertilizer, showed a 2-fold higher immobilization of 15N into microbial biomass in the high fungal biomass soil. However, only 3% of total 15N was found in the microbial biomass 2 weeks after the mineral fertilization. Most of the recovered 15N was in the plants (approximately 25%) or in the soil organic matter (approximately 15%). Our main experiment confirmed the assumption of lower N losses in a soil with higher fungal biomass. The additional 15N experiment showed that higher fungal biomass is probably not the direct cause of higher N immobilization, but rather the result of low nitrogen availability. Both experiments confirmed that higher fungal biomass can be considered as an indicator of higher nutrient retention in soils

Spatial predictions of maize yields using QUEFTS – A comparison of methods

Using fertilisers is indispensable for closing yield gaps in Sub Saharan Africa. Current fertiliser recommendations, however, are often blanket recommendations which do not take spatial variation in soil conditions within a region or country into account. Soil maps can potentially support fertiliser recommendations at a higher spatial resolution. The QUantitative Evaluation of the Fertility of Tropical Soils (QUEFTS) model is a decision support tool that predicts crop yields as an indicator of soil fertility and can be used to evaluate yield responses to fertilisers. It was designed for field level output and runs on field-specific soil information. The aim of this study was to compare two methods for developing maps of QUEFTS output, i.e. maize yield and the yield-limiting nutrient, with Rwanda as a case study. We used a database containing soil analysis results of 999 samples collected across Rwanda. Transfer functions were applied to predict the required P-Olsen and Exchangeable K input for QUEFTS based on the soil data. For the “Calculate-then-Interpolate” (CI) method, transfer functions and QUEFTS were applied to point data, and the final output was then interpolated using random forest modelling. For the “Interpolate-then-Calculate” (IC) method, maps of the soil parameters were developed first, before applying calculations. Implications of the chosen method (i.e. CI or IC) on QUEFTS predictions on a national scale were evaluated using set-aside locations. Results showed low precision and accuracy of QUEFTS maize yield predictions across Rwanda. The CI method performed better in predicting QUEFTS yield and yield-limiting nutrient than the IC method. Correlations between mapped yield predictions and predictions on set-aside evaluation locations were similar for the CI (r = 0.444) and IC (r = 0.439) methods. The poorer performance of the IC method was mostly due to overestimation of yields, which was most likely caused by the effect of smoothing on the soil maps used as input for QUEFTS. We conclude that the CI method is the preferred method for spatial application of QUEFTS

Dissolved Organic Nitrogen as proxy for mineralisation?

Strong relationships between Extractable Organic N (EON) and N mineralization suggest that they can be used to assess the N mineralization potential of soils. EON is often used to proxy Dissolved Organic Nitrogen (DON), which is assumed to play an intermediate role in N mineralization. We tested whether EON relates to N mineralization and how this relation is affected by the extraction methodology used. We synthesized results from 171 papers on correlation coefficients between EON and N mineralization using meta-analysis. EON was positively related to N mineralization, and its size explained 14-65% of the variation in N mineralization. Best results were obtained with hot CaCl2 and acid KMnO4 as extractants. EON extracted with alkali solutions explained less of the variation in N mineralization than the other soil tests. There was no direct relationship between the intensity of extraction and the performance of EON as predictor for N mineralization. When EON levels are used to make fertilizer-N recommendations, the extraction methodology used to develop the above described relationship is closely followed

Role of soil fungi in nitrogen cycling

It is often suggested but so far not substantiated that soils with a higher fungal/bacterial (F/B) ratio retain nitrogen better. If this were true, it would be wise for farmers who want to reduce the environmental impact to increase this F/B ratio. We did field studies on experimental plots and on commercial farms and found that lower fertilizer rates led to higher fungal biomass in grassland soils. In incubation studies with intact soil columns we showed that soil with a higher fungal biomass had lower leaching and denitrification rates because of lower mineralization and nitrification rates while maintaining production levels