Localized application of phosphorus and ammonium improves growth of maize seedlings by stimulating root proliferation and rhizosphere acidification
Introduction
Phosphorus (P) is often the first limiting nutrient in both acid and calcareous soils due to its low availability to plants. The total amount of P in the soil may be high, ranging from 0.02% to 0.5% (w/w), with an average of about 0.05% (w/w); however, available P concentrations in soil solution range only from 0.1 to 10 μM, which is below the sufficient P concentration for optimal plant growth (Barber and Peterson, 1995, Hinsinger, 2001). Only a small proportion of applied P can be absorbed by plants in the first growing season (Holford, 1997, Schachtman et al., 1998, Vance et al., 2003) because the applied P can be easily fixed or adsorbed by soil particles in both acid and calcareous soils. In addition, the diffusion rate of P in soils is very low, with the diffusion coefficient of 10−12 to 10−15 m2 s−1 (Schachtman et al., 1998). Limited phosphate (Pi) resources in the world and poor availability of Pi in soil may result in a potential Pi crisis in agriculture in the near future because it is estimated that inexpensive phosphate rock reserves (i.e. those easy to extract and process) will be depleted in the next 60–90 years (Vance et al., 2003, Cordell et al., 2009).
Root growth and root system establishment play a crucial role in the capture and acquisition of resources such as water and nutrients, thus strongly influencing nutrient use efficiency by crops. Root systems are highly plastic in their development, with root architecture adapting to the specific environmental conditions (Williamson et al., 2001, Shen et al., 2005, Shu et al., 2007). Geometry and morphology of roots are important in maximizing P uptake because root systems with higher ratios of surface area to volume more effectively explore a given volume of soil (Lynch, 1995).
Plant roots can respond to heterogeneous nutrient supply in their environments. The number of lateral roots in barley was greatly stimulated by localized supply of nitrate and ammonium or P (Drew, 1975). When soluble phosphate was applied in a band, roots extensively proliferated in this P-enriched soil (Havlin et al., 2005). In N- and P-rich soil patches, lateral root proliferation was one of the most important root responses (Robinson, 1994, Robinson, 2001). Hence, manipulation of root morphology may result in improved plant growth and P use efficiency.
Phosphorus absorption by maize could be enhanced by combined application of P and nitrogen (N) (Cole et al., 1963, Thien and Mceff, 1970), with P uptake rates highly related to the N supply, and N pretreatment increasing the rate of P translocation to shoots 5- to 10-fold (Cole et al., 1963). Nevertheless, in tomato, P limitation reduced tissue N concentration possibly in relation with a decrease in leaf cytokinin levels (De Groot et al., 2003). Duncan and Ohlrogge, 1958, Duncan and Ohlrogge, 1959 and Miller and Ohlrogge (1958) found that N and P banded together increased P absorption due to extensive proliferation of roots in the fertilized zone; neither N nor P applied alone caused such an increase in root growth. Therefore, the magnitude of root proliferation in the localized nutrient-rich patches was highly dependent on the specific nutrient composition and their interaction.
Rhizospere processes such as root-induced changes in pH or redox potential and root exudate release (Marschner, 1998) play a key role in nutrient acquisition. Rhizosphere chemistry can be significantly changed according to the form of N taken up: ammonium supply may reduce rhizosphere pH through promoting proton release, whereas nitrate supply may increase rhizosphere pH through releasing anion OH− (Marschner, 1995, Taylor and Bloom, 1998, Hinsinger et al., 2003). In calcareous soils with relatively high pH, ammonium-induced acidification in the rhizosphere could lower the pH, and thus improve the nutrient uptake, particularly for sparingly available P such as Ca phosphates that are the dominant forms of P fixed in such soils. Taken together, manipulation of rhizosphere processes by stimulating (i) root proliferation and/or (ii) rhizosphere acidification through optimizing N forms and P input may be an effective approach to improving plant growth and P use efficiency.
The objective of the present study was to test (i) whether localized supply of P combined with ammonium can stimulate plant growth and nutrient uptake by altering root morphology and rhizosphere acidification in maize grown in a calcareous soil in the field, and (ii) examine the effects of localized supply of P plus ammonium on root distribution in the soil profile.
Section snippets
Field experiment
Field experiments were conducted to characterize the effects of localized P supply in combination with ammonium on maize root growth and nutrient uptake in an intensive farming system in North China in 2007 and 2008. The trial site was located at the research station of China Agricultural University in Shangzhuang, North China, near the Pacific Coast around 40° N latitude, 116° E longitude. Maize is widely grown in this region. The annual average temperature in the region ranges from 11 to 13
Plant growth
Localized supply of ammonium and P had a significant influence on plant growth, chlorophyll status, total leaf area, and biomass of plants in different growth stages. The chlorophyll (SPAD) reading on the youngest fully developed leaves increased from 26 DAS to 39 DAS, and from 51 DAS to 71 DAS, but was not different between 39 and 51 DAS (Table 2). The chlorophyll reading was significantly higher in the treatment with localized ammonium and P supply (NLPL) than NBPL (N broadcast and P
Effect of localized N and P supply on plant growth and root development
Large amounts of soil nutrients can be captured by expanding root absorption surface due to root proliferation. The nutrient-rich patches significantly stimulate root proliferation, including increased lateral root number and lateral root length (e.g. Drew, 1975, Robinson, 1994, Zhang and Forde, 1998, Forde, 2002, Linkohr et al., 2002, Misson et al., 2005, Schachtman and Shin, 2007), underpinning a potential approach to improving plant growth and nutrient use efficiency through manipulation of
Acknowledgements
This study was supported by National Natural Science Foundation of China (30890133, 30925024, 30871591) and the innovative group grant of the NSFC 30821003), the Ministry of Science and Technology of China (2006BAD25B02, 973-2007CB109302) and National Department Public Benefit Research Foundation ((200803029-01-02). We thank Prof. V. Römheld (The University of Hohenheim, Germany) for his comments and suggestion for the study.
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