The introduction of hybrid walnut trees (Juglans nigra × regia cv. NG23) into cropland reduces soil mineral N content in autumn in southern France
The introduction of trees in cropland may be a way to improve the mineral nitrogen (N) use efficiency since tree roots can intercept N leached below the crop rooting zone and recycle it as organic N. The aim of this study was to determine soil mineral N (SMN) and total N (STN) contents after 14 years of hybrid walnut tree growth in an agroforestry system. Soil cores were collected and analyses in mid-autumn 2009, in intercropped agroforestry (AF), pure tree (FC) and sole crop control (CC) plots. The SMN was significantly reduced in AF compared to CC (64, 58 and 51 % of reduction at 0.2, 1 and 2 m depth respectively). In the top 1 m of soil, the stock of SMN was 77.7 kg N ha⁻¹ in CC versus 32.8 kg N ha⁻¹ in AF. Trees in AF developed deeper fine roots than in FC, likely involved in the reduction of SMN when compared to CC. Despite this quantitative reduction, trees also progressively modified the form of mineral N in soil by decreasing the percentage of nitrate (NO₃ ⁻) in SMN, particularly in FC compared to CC, while AF was intermediate. The STN was not significantly different between AF and CC; but was higher in FC in the top soil, probably due to weeds and superficial tree root biomasses. Our results suggest that the introduction of hybrid walnut trees into cropland may be an efficient practice to reduce the potentially leachable N by winter rainfall.
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