Field measurement of lupin belowground nitrogen accumulation and recovery in the subsequent cereal-soil system in a semi-arid Mediterranean-type climate

In situ 15N-labelling was used to provide a quantitative assessment of the total contribution of lupin (Lupinus angustifolius) to below-ground (BG) N accumulation during a growing season under field conditions, and to directly trace the fate of the lupin BG N in the next season, including quantifying the N benefit from lupin to a following wheat (Triticum aestivum) crop. The experiments were conducted at two sites, both experiencing a semi-arid Mediterranean-type climate in the wheat-growing region of Western Australia but with differing soil types, a deep sand (Moora) and a sand-over-clay shallow duplex soil (East Beverley, EB). Lupin shoot and root dry matter and total plant N accumulation, proportional dependence on nitrogen fixation and grain yield were greater at the deep sand site than the duplex soil site, although there was a similar proportion of shoot N to estimated total BG N at both sites. The proportion of total plant BG N decreased from the vegetative stage (42-51%) to peak biomass (25-39%) and maturity (23-34%). From 56-67% of BG N on the deep sand and 74-86% on the duplex soil was not recovered in coarse roots (>2 mm) or as soluble N, but was present in the insoluble organic N fraction. There was evidence for cycling of lupin root-derived N into soil microbial biomass and soluble organic N during lupin growth (by the late vegetative stage), but no evidence for leaching of legume derived BG N during the lupin season. Estimates of fixed N input BG were at least four times greater if based on total lupin BG N rather than on N recovered in coarse roots (>2 mm). There were no apparent losses of lupin BG N during the summer fallow period subsequent to lupin harvest at either site. Also, immediately prior to sowing of wheat there were similar proportions of lupin BG N in the inorganic (20-25%) and microbial biomass (6-9%) pools at both sites, with the majority of BG N detected in the <2 mm fraction of the soil column. However, the proportion of residual lupin BG N estimated to benefit the aboveground wheat biomass was relatively low, 10% on the deep sand and only 3% on the shallow duplex. Some (14%) residual lupin BG N was leached as nitrate to 1 m on the deep sand compared to 8% of residual lupin BG N leached to the clay layer (0.3 m) on the shallow duplex. About 27% of the residual lupin BG N on the deep sand at Moora had apparently mineralised by the end of the succeeding wheat season (i.e. recovered either in the wheat shoots, as inorganic N in the soil profile or as leached nitrate) compared to only 12% at EB. There was an unaccounted for large loss of residual lupin BG N (50%) from the duplex soil at EB during the wheat season, postulated to be chiefly via denitrification. At both sites after the wheat season a substantial proportion (32-55%) of legume derived BG N was still present as residual insoluble organic N, considered to be an important contribution to structural and nutritional long-term sustainability of these soils. © 2007 Springer Science+Business Media B.V.A. M. McNeill and I. R. P. Filler

Using a Tri-Isotope (13C, 15N, 33P) Labelling Method to Quantify Rhizodeposition

Belowground (BG) plant resource allocation, including roots and rhizodeposition, is a major source of soil organic matter. Knowledge on the amounts and turnover of BG carbon (C), nitrogen (N), and phosphorus (P) in soil is critical to the understanding of how these elements cycle in soil-plant system. However, the assumptions underlying the quantification and tracking of rhizodeposition using isotope labeling methods have hardly been tested. The main objectives of this chapter were to (i) review the different plant labeling techniques for each of the three elements; (ii) describe a novel method for the simultaneous investigation of C, N, and P rhizodeposition in sand; and (iii) test the methodological assumptions underlying quantification of rhizodeposition. Stable 13C and 15N isotopes were widely used to study rhizodeposition of plants either separately or in combination, while P radioisotopes (32P, 33P) were used to investigate root distribution. The combination of the 13CO2 single-pulse labeling with the simultaneous 15N and 33P cotton-wick stem feeding effectively labeled Canavalia brasiliensis roots and facilitated the estimation of rhizodeposited C, N, and P input from root systems. However, the isotope distribution was uneven within the root system for all three elements. Additionally, we observed a progressive translocation from shoot to roots for 15N and 33P over 15 days after labeling, while the 13C tracer was diluted with newly assimilated non-enriched C compounds over time. Younger root sections also showed higher specific activities (33P/31P) than the older ones. The relatively high 33P radioactivity recovered in sand right away at the first sampling was attributed to an artifact generated by the stem feeding labeling method. Overall, our results suggest that the assumptions underlying the use of isotope methods for studying rhizodeposition are violated, which will affect the extent of quantification of rhizodeposition. The consequences of nonhomogeneous labeling of root segments of different age require further investigation. The use of a time-integrated isotopic composition of the root is recommended to not only account for temporal variation of isotopes but also to improve the method of quantifying plant rhizodeposition

Ecological principles underlying the increase of productivity achieved by cereal-grain legume intercrops in organic farming

World population is projected to reach over nine billion by the year 2050, and ensuring food security while mitigating environmental impacts represents a major agricultural challenge. Thus, higher productivity must be reached through sustainable production by taking into account climate change, resources rarefaction like phosphorus and water, and losses of fertile lands. Enhancing crop diversity is increasingly recognized as a crucial lever for sustainable agro-ecological development. Growing legumes, a major biological nitrogen source, is also a powerful option to reduce synthetic nitrogen fertilizers use and associated fossil energy consumption. Organic farming, which does not allow the use of chemical, is also regarded as one prototype to enhance the sustainability of modern agriculture while decreasing environmental impacts. Here, we review the potential advantages of eco-functional intensification in organic farming by intercropping cereal and grain legume species sown and harvested together. Our review is based on a literature analysis reinforced with integration of an original dataset of 58 field experiments conducted since 2001 in contrasted pedo-climatic European conditions in order to generalize the findings and draw up common guidelines. The major points are that intercropping lead to: (i) higher and more stable grain yield than the mean sole crops (0.33 versus 0.27 kg m(-2)), (ii) higher cereal protein concentration than in sole crop (11.1 versus 9.8 %), (iii) higher and more stable gross margin than the mean sole crops (702 versus 577 a,not signaEuro parts per thousand ha(-1)) and (iv) improved use of abiotic resources according to species complementarities for light interception and use of both soil mineral nitrogen and atmospheric N-2. Intercropping is particularly suited for low-nitrogen availability systems but further mechanistic understanding is required to propose generic crop management procedures. Also, development of this practice must be achieved with the collaboration of value chain actors such as breeders to select cultivars suited to intercropping