FIELD OBSERVATIONS ON NITROGEN CATCH CROPS - II - ROOT LENGTH AND ROOT LENGTH DISTRIBUTION IN RELATION TO SPECIES AND NITROGEN SUPPLY

Nitrogen catch crops help to reduce the loss of nitrogen from arable c ropping systems during autumn and winter. The ability of catch crops t o absorb nitrogen from the soil profile is affected by rate and depth of colonization of the soil by roots. The aim of the current work was to analyze total root length and root length density of catch crops in relation to above ground growth, nitrogen supply and crop species. In two field experiments roots were sampled with an auger. Experimental factors included crop species (winter rye, Secale cereale and forage r ape, Brassica napus ssp. oleifera (Metzg.) Sinsk., or oil radish, Raph anus sativus spp. oleiferus (DC.) Metzg.), two sowing dates S1 and S2 (end of August and three weeks later) and two nitrogen treatments: NO, no nitrogen applied, and N1, nitrogen applied at non-limiting rate. T he natural logarithm of the total root length, measured in the top 40 cm, L0-40 (km m(-2)), was linearly related to natural logarithm of the dry weight of the shoot, W (g m(-2)). There was no effect of species or sowing date on this relation. For a given W, N1 treatments showed l ower values of L0-40 than NO treatments. The decline in root length de nsity, D (cm cm(-3)), with depth, X (cm), was described with the funct ion In D = In Do - qX, where Do is the value of D at zero depth and q the linear coefficient. Do was linearly related to L0-40 without effec t of species, time of observation or N supply. The ratio D-O/q, an est imate of the absolute root length, was 1.24 x L0-40 Together the relat ions enable estimates to be made of total root length and of root leng th distribution with depth using shoot dry weight of catch crops and i ts change with time as input. The generation of such estimates of root distribution is necessary for model studies in which the efficacy of catch crops to prevent N losses is evaluated in relation to sowing dat es, distribution of N in the soil profile and the distribution of rain fall in the season.