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Purpose: Polyphosphate (poly-P) fertilizers were proposed to increase the low phosphorus use efficiency (PUE) of orthophosphate (ortho-P) fertilizer but empirical data is inconsistent. Our study is aimed at investigating how soil properties, application time, and growth duration affect their phyto-availability. Methods: Olsen-P dynamics were monitored using six soils amended with either ortho-P or poly-P. Subsequently, two contrasting soils (sandy and loamy) were selected to estimate P availability to plants using wheat as a model plant. Mechanistic modelling was used to interpret P dynamics in these experiments. Results: For poly-P amended soils, Olsen-P increased continuously, while using ortho-P fertilizer the highest values were attained right after application and then monotonously decreased afterward. Olsen-P dynamics varied greatly among soils depending on their sorption capacity and related properties. In sandy soils, Olsen-P was consistently higher for ortho-P. In highly calcareous soils, Olsen-P was higher for poly-P after three weeks. Plant experiments confirmed a greater uptake of poly-P over ortho-P in loamy soil and extended trials (4 months) but not in sandy soils or short-term trials (4 weeks). Geochemical modeling suggested that complexation of soil Ca by poly-P reduced long-term P immobilization. Conclusions: Soil properties and growth duration affect the efficiency of ortho-P vs. poly-P fertilizers. In clayey soils and extended growth duration, poly-P fertilizer can enhance PUE due to its prolonged phyto-availability as compared to ortho-P. In sandy soils and short growth duration poly-P has little advantage. The complexation of Ca by poly-P may have caused the increase of P phyto-availability. [ABSTRACT FROM AUTHOR]

There are growing evidence that nitrogen (N) recommendation based on the expected yield concept developed by Stanford in 1973 lacks in agronomic principles, despite it widespread use worldwide. In Brazil, the main sugarcane producer worldwide, for example, a fixed N factor of 1 kg N per Mg−1 of stalk is used. However, literature demonstrates that sugarcane responsiveness to N is much higher in sandy soils rather than in clayey soils, whereas the recommended N is usually higher in clayey soils because of its improved yield potential. We investigated 146 response curves of sugarcane (Saccharum spp.) to N to define a better approach for determining N rates instead of using exclusively the expected yield concept. First, we found no correlation between the economically optimal N rate (Nopt, kg ha−1) and the yield (Mg ha−1) obtained in Nopt. Second, we calculate the N requirement (Nreq, kg N Mg−1 stalk) as the quotient between Nopt and the yield, for each response curve. There was a negative correlation between yield at control plot and Nreq, demonstrating that higher N rates are required to maximize yield in sandy (low yield) sites. The Nreq was 1.3, 0.9, 0.7 and 0.6 kg N Mg−1 stalk for expected yields < 69, 69–84, 84–102, and > 102 Mg ha−1, respectively, differing from the standard 1 kg N Mg−1 stalk currently used. The method proposed here is based indirectly on the N-supplying power of the soil and should be tested for other crops to deliver an improved N recommendation system. [ABSTRACT FROM AUTHOR]

Aims: Shrub encroachment into grasslands alters organic carbon pools and patterns of decomposition. Here, we quantify the influence of plant life-form patches occuring on contrasting geomorphic substrates on (1) soil-litter mixing (SLM) and litter decomposition and 2) the development of soil aggregates and soil-microbial films on litter. Methods: A factorial experiment in a shrub-invaded arid grassland tested the hypotheses that SLM would promote decomposition, and that development of soil films and aggregates would be inversely related to grass cover and positively related to soil surface clay content. Litterbags containing shrub (Prosopis velutina) leaf litter were deployed on a sandy Holocene-age soil and on a clayey Pleistocene-age soil under and away from shrubs on areas with low and high grass cover. Results: SLM accelerated decomposition, with highest rates on sandy soils in inter-shrub areas with low grass cover. Decomposition on clayey soils was slower, despite more rapid development of soil films. Soil-microbial film formation in bare ground placements exceeded that in shrub or grass placements and was greater on sandy Holocene-age soils compared to clayey Pleistocene-age soils. Conclusions: Vegetation structure and geomorphology should be considered when modeling decomposition dynamics in systems with low plant cover and high rates of aeolian and fluvial soil movement. [ABSTRACT FROM AUTHOR]

Aims: While patterns of variation in woody plant aboveground traits related to disturbance and resource availability in savanna ecosystems are fairly well understood, dimensions of variation in belowground traits remain poorly understood. We investigate how sapling coarse root (>2 mm diameter) traits and belowground storage organs vary with respect to differing disturbance regimes (i.e. fire or herbivore dominated) and soil fertility in Southern African savannas. Methods: We examined how sapling rooting strategies (type of rooting system, storage organs, root allocation) as well as root total non-structural carbohydrate concentrations [TNC], root N and C:N of 69 common woody savanna and forest species at 16 sites vary with respect to differing disturbance regimes and soil fertility. Results: We found that root [TNC] and the occurrence of storage organs were highest while root N was lowest for species growing at fire-driven sites, compared to herbivore-driven sites and competition-controlled forest species. Allocation to belowground biomass was higher at fire-dominated sites but root volume:stem basal area did not differ between disturbance regimes. None of the measured traits were found to differ between nutrient rich clayey soils and nutrient poor sandy soils. Conclusions: Our results suggest that disturbance related controls are important drivers of savanna belowground traits. [ABSTRACT FROM AUTHOR]

Hydraulic lift can be a desirable characteristic for cotton growing in areas that experience frequent drying of the topsoil. This study employed specialized soil columns to test if cotton can hydraulically lift water and whether this hydraulically lifted water can facilitate P acquisition from the dry topsoil of a clay and a sandy soil. The role of subsoil P placement on cotton growth was also tested. By using a Theta probe, hydraulic lift was detected in the clay soil for 7 days after water was withheld from the topsoil. However, in the sandy soil, a significant overnight increase in soil water was only detected for one day for the treatment with subsoil P application. Where P was applied to the topsoil, and water was withheld, cotton roots showed significant root P uptake from the sandy soil, but negligible P uptake from the clay soil. Irrespective of topsoil watering, the higher root length density in the subsoil of the clay soil accounted for the pronounced response to deep P placement. This study demonstrated that soil texture affects the amount and longevity of hydraulic lift from cotton roots. Hydraulic lift detected in the clay soil did not aid P acquisition from dry surface soil by cotton plants. [ABSTRACT FROM AUTHOR]