A study of the quality factors affecting the short-term decomposition of field vegetable residues

Experiments were carried out to characterise the composition and decomposition of 13 vegetable crop residues. There was a considerable range of composition, with 55% of the dry matter being water-soluble in onion compared with only 15% in wheat. The range of carbon/nitrogen ratio in the vegetable crops was narrow, between 9 and 24, compared with a C/N ratio of 58 in wheat. The decomposition of the residues could be described by the single-exponential equation Y = a + be−k(t − d), where Y is the proportion of residue remaining at any thermal time day degrees above 0 °C, a is the proportion of residue remaining at the end of the experiment, b is the corresponding proportion of the degradable fraction, k is the rate constant for the decomposition process and d is a delay factor (in units of thermal time) which delays the start of the decomposition process. Y, a and b are all expressed as percentages on a weight basis and, by definition, a + b must equal 100 at t = 0. The value of d is assumed to equal zero for all residues except those from leafy brassicas, for which it was set at 100. The half-life of crop residue decomposition ranged from 145 day deg for onion to over 1400 day deg for wheat. The decomposition rate constant (k) was related to the C and N contents (r = 0.84, p = 0.001), and the a parameter to the crop's lignin content (r = 0.74, p = 0.004)