Effect of Soil Salinity and Nitrogen Availability on Photosynthate Partitioning and Growth of Wheat

Abstract

Salt-affected soils comprise about 10% of the world’s arable lands. Of a total of 380 million hectares of saline soils, 140 million hectares are highly saline. In Pakistan alone, about 6.7 million hectares are affected by salinity of different extent and the problem is getting worse due to rapid salinization of originally productive soils mainly because of the agroclimatic condition including high temperatures and surface irrigation. These soils are characterized by high salt contents (sometimes high pH as well) and low content of organic matter and nitrogen (N). Widely divergent approaches have been proposed and practiced to use these soils for agricultural activity. However, nutrient constraints that are characteristics of these soils have generally received little attention. Information is particularly lacking on: i) forms and availability to plants of N under saline conditions and ii) the effect of N supply on photosynthate partitioning and the resultant changes in soil micro flora and microbial fuctions. Therefore, the objectives of the studies reported here were: i) Quantification and chemical characterization of N available for plant uptake in saline soils. ii) Plant uptake, assimilation of NH4+, NO3- -N and NH4+, NO3--N with a view to select appropriate form of N for application under saline conditions, iii) Photosynthate partitioning (using 14C labeling of plants) under saline conditions and the role of rhizodeposition in mitigating negative effects of salinity. Under laboratory conditions, soil samples amended with 0.5% plant material (sesbania aculeata) and salinized to ECe 7, 9, and 18 dSm-1 (original ECe was 5.0 dSm-1) were incubated at 3 moisture levels (15, 30 and 45% w/w) and three temperature regimes i.e., 20, 30 and 40 oC for 8 weeks and sub-samples studied for i) NH4-N, ii) NO3-N, iii) mineralizable N, iv) 6N HCl hydrolysable N, v) distribution of hydrolysable N in different fractions, and vi) alkali labile N. ammonification of organic N (as determined by the accumulation of NH4-N in soil) increased with moisture and temperature and decreased with salinity. The content of mineralizable N remained higher under high moisture conditions, while high salinity and temperature had a variable and negative effect. Complete loss of NO3-N observed during incubation of soil samples was attributable to denitrification. Further analyses of soil samples revealed significant quantities (82-92 ug g-1 soil) of hydrolysable N in alkali labile fraction. This represented 14-15% of the organic N in soil and was significantly correlated with potentially mineralizable N determined by anaerobic incubation. The results suggested a common origin of the two forms of N. hence alkali labile N could possibly be used as an index of plant available N. In the greenhouse, a pot experiment was conducted to study the effect of form and availability of N on wheat. Nitrogen was applied 15N- labeled NH4 or NO3 in the form of ammonium sulphate, potassium nitrate or ammonium nitrate; DCD was applied as nitrification inhibitor in one of the treatments. Among the different sources of N, ammonium sulphate proved better in affecting the growth of wheat; nitrification inhibition had a depressing effect. A decrease due to salinity was observed in the net amount of N harvested by plants and thus the dry matter accumulation. The studies suggested that a good mix of NH4 and NO3 may be more appropriate for wheat under saline conditions. Under hydproponic condition, salinity of the rooting medium had a significant negative effect on growth of wheat; the effect was more serve in the presence of NH4 than NO3. The increase in NH4 concentration in the medium led to a consistent decrease in biomass accumulation. Results of 14C pulse labeling experiment showed a positive effect of NH4-N on rhizodeposition and a negative effect on respiration; reverse was true for NO3-N. Salinity of the rooting medium had a positive effect on rhizodeposition and a negative effect on the amount of 14C respired. It appeared that inhibition of nitrification under saline conditions will have both direct and indirect effects on portioning of photosythates to the root zone and hence the rhizospheric microbial function. Experiments conducted under greenhouse and filed conditions to study the effect of organic amendment on growth of wheat suggested that native mineral N is an important determinant of the plant responses to applied N. Sesbania that has a higher N content and narrow C/N ratio had some positive effect on wheat, but application of kallar grass (wide C/N ratio) significantly retarded plant growth either due to N immobilization or release of growth inhibitory factors. It was observed that plant residues having high N concentration and that are easily decomposable can substantially replace chemical fertilizers like urea. Chemistry of photosystem-II was not significantly affected by different soil treatments.