The productivity and sustainability of the prevailing crop production systems are being challenged throughout the Indo-Gangetic Plains. Limiting water resources, depletion of soil fertility, social changes and economic developments drive the current modification of the crop portfolio, reflected in its spatial-temporal patterns and of cultivation practices. In Nepal, this concerns particularly the rice-wheat annual double cropping system, which is the dominant food crop rotation in both the subtropical lowland as well as the temperate Himalayan mid hills of Nepal. As a results of continuing urbanisation and shifting consumer preferences, a drive to replace of wheat with high-value vegetables during the cold dry season is gaining momentum, in the peri-urban fringes., simultaneously, emerging water shortages are preventing permanent soil flooding during the monsoon season, leading to partial substitution of lowland rice by less water-consuming upland crops. Such system shifts and associated changes in soil aeration status are altering the nutrient availability, while increasing the crop demand for the critically limiting micronutrients boron (B) and zinc (Zn).
Therefore, compared the B and Zn levels in the traditional rice- based system (under anaerobic condition), in the water-saving maize-based system (aerobic conditions) with both conventional winter wheat and the emerging vegetables as rotation crops. Under controlled conditions in a dysfunctional greenhouse and under field conditions at two representative production sites and soil types (e.g. Acrisols in Kavre in the mid-hills of Nepal and a Fluvisols in Chitwan in the lowland), determined were(1) differential effects of system shifts on the soil supply and crop demand of B and Zn (diagnosis trials), (2) the effects of applying mineral B and Zn fertilizers on yields and economic returns of wheat vs. cauliflower and tomato (response trials), and (3) longer-term carry-over effects of a one-time application of soil B and Zn on biomass accumulation and nutrient uptake by maize (residual effect trials). Inclusion of an aerobic soil phase (e.g. maize instead of rice) resulted in declining soil C and N contents and consequently negatively affected dry matter accumulation and wheat grain yield. Concurrently, the shift from wheat to cauliflower and tomato increased the demand for B and Zn, and these vegetables showed deficiency symptoms at both sites and in both soil types. Particularly the B concentrations in the biomass of non- amendments crops were always below the critical limits of <10 (wheat), 21 (cauliflower) and 23 mg B kg-1 (tomato). In wheat, the application of Zn tended to increase yields under field conditions, while a B application showed no significant effect, irrespective of the site or soil type. On the other hand, biomass accumulation, nutrient uptake, and economic yield of cauliflower and tomato increased with B (and Zn) applications, but response attributes were unaffected by changes in soil aeration status. These responses were generally more pronounced in the lowland than the mid-hill sites, while overall yields of wheat and temperate vegetables were higher in the cool mid-hills than in the subtropical lowland. Despite low application rates of 2.2-4.0 kg ha-1 of Zn or B, positive residual effects on subsequent non -fertilized maize were observed with Zn in the Acrisols and with B in both soil types. Soils in larger parts of Nepal are low in available B and Zn. A shift towards aerobic cultivation in the wet season will reduce soil C and N contents and concomitantly the supply of B and Zn. At the same time, the current shift from wheat to vegetables increases the crops’ demand for B and Zn. While the application of B and Zn fertilizers can moderately improve the performance of the traditional rice-wheat rotation, with a shift towards vegetable cropping, B and Zn applications become imperative to sustain production. Both the public and the private sectors will increasingly be challenged to develop and make available B- and Zn-containing fertilizer formulations that respond to the changing needs of the emerging production systems. These findings are also pertinent in other environments and for other farming communities in the Indo-Gangetic Plains and the Himalayan foot-hills beyond Nepal
