Abstract
Many arable lands have accumulated large reserves of residual phosphorus (P) and a relatively large proportion of soil P is less available for uptake by plants. Root released organic anions are widely documented as a key physiological strategy to enhance P availability, while limited information has been generated on the contribution of rhizosphere organic anions to P utilization by crops grown in agricultural soils that are low in available P and high in extractable Ca, Al, and Fe. We studied the role of rhizosphere organic anions in P uptake from residual P in four common crops Triticum aestivum, Avena sativa, Solanum tuberosum, and Brassica napus in low- and high-P availability agricultural soils from long-term fertilization field trials in a mini-rhizotron experiment with four replications. Malate was generally the dominant organic anion. More rhizosphere citrate was detected in low P soils than in high P soil. B. napus showed 74–103% increase of malate in low P loam, compared with clay loam. A. sativa had the greatest rhizosphere citrate concentration in all soils (5.3–15.2 μmol g−1 root DW). A. sativa also showed the highest level of root colonization by arbuscular mycorrhizal fungi (AMF; 36 and 40%), the greatest root mass ratio (0.51 and 0.66) in the low-P clay loam and loam respectively, and the greatest total P uptake (5.92 mg P/mini-rhizotron) in the low-P loam. B. napus had 15–44% more rhizosphere acid phosphatase (APase) activity, ~0.1–0.4 units lower rhizosphere pH than other species, the greatest increase in rhizosphere water-soluble P in the low-P soils, and the greatest total P uptake in the low-P clay loam. Shoot P content was mainly explained by rhizosphere APase activity, water-soluble P and pH within low P soils across species. Within species, P uptake was mainly linked to rhizosphere water soluble P, APase, and pH in low P soils. The effects of rhizosphere organic anions varied among species and they appeared to play minor roles in improving P availability and uptake.
Highlights
In order to produce enough food to feed the increasing global population, large amounts of mineral phosphorus (P) fertilizers have been used
We carried out an experiment using a mini-rhizotron culture system as described by James et al (1985) and selected two dicot and two monocot widely cultivated crops (Triticum aestivum, Avena sativa, Solanum tuberosum and Brassica napus) to investigate the contribution of root-exuded organic anions to improving P uptake in agricultural soils low in P availability. For these four crops grown in three soils obtained from long-term fertilization field plots in Norway, we addressed three hypotheses: (1) Low P availability will stimulate plant roots to release more organic anions and acid phosphatase (APase) to rhizosphere soil; (2) the amounts of rhizosphere organic anions and APase will have positive correlations with rhizosphere plant-available P fractions and P uptake by plants in low P soils; (3) different crops will show differences in root released organic anions and APase in terms of using residual P from agricultural soils
Our results compared well with the findings of Nazeri et al (2014), who used agricultural soil. These findings suggested that rhizosphere organic anions may play a minor role in improving soil P availability and plant P uptake
Summary
In order to produce enough food to feed the increasing global population, large amounts of mineral phosphorus (P) fertilizers have been used. In Norwegian agricultural soils, the average P surplus has been reduced by nearly 50% since 1985, the surplus is still about 15 kg P ha−1 year−1 (Eurostat, http://ec.europa.eu/eurostat/web/ agri-environmental-indicators/farm-management-practices). Many arable lands have accumulated large reserves of residual P: In Norway, a relatively large proportion of cultivated soils is classified as high to very high in extractable P (Singh and Subramaniam, 1996). 3–25% of total P was extractable in ammonium lactate (PAL), which is an estimate of the plantavailable P, whereas 34–68% of P was sorbed to Al, Fe, and Ca. In addition, organic P accounted for 17–51% of total P in different Norwegian soils (Singh et al, 2005). Improving the utilization of residual P accumulated in agricultural soils would help to reduce P fertilizer application and environmental stress. Studies on how to mobilize the less-available residual P and improve P-acquisition efficiency are needed
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