Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 237

residue with a C:N ratio 30:1 (e.g., cereal straw) typically results in tempo-

rary immobilization of nutrients by soil microbes before nutrients are
released in plant-available, inorganic forms, whereas application of residue
with a C:N ratio 20:1 (e.g.,immature legume material) increases soil concen-

trations of plant-available nutrients as soon as environmental conditions
allow enough microbial activity (Stevenson, 1986, pp. 164-6). Because of its
critical importance in plant nutrition, much of the research relevant to
residue decomposition and nutrient release has focused on N.However, P and
S release from crop residue can follow temporal patterns similar to N
(Stevenson, 1986, pp.268, 301).
Because residue decay and nutrient transformations require time, it is not
surprising that increases in soil inorganic nutrient concentrations often occur
more slowly following crop residue application than synthetic fertilizer appli-
cation. For example, in a maize field in Kentucky treated with 15 N-labeled
ammonium nitrate and hairy vetch residue, Varco et al . (1993) found the per-
centage of soil inorganic 15 N derived from the fertilizer exceeded that from
the green manure for 30 days after treatments were applied in one year, and
for 45 days in a second year. Similarly, in a rice field in California, Westcott &
Mikkelsen (1987) observed that soil inorganic N levels were lower for the first
48 days following incorporation of purple vetch, compared to application of
ammonium sulfate containing the same amount of N.
Data concerning plant N uptake also suggest that crop residue can function
as a slow-release nutrient source, compared to synthetic fertilizer applied in a
single dose at the start of the growing season.In a greenhouse pot experiment
using ammonium sulfate and Sesbania aculeata residue labeled with 15 N,Azam,
Malik & Sajjad (1985) observed that after five weeks of growth maize had
recovered 20% of the labeled N from the fertilizer, but only 5% from the green
manure.In a field experiment conducted in a semiarid area of South Australia,
Ladd & Amato (1986) found that 17% of the 15 N label in a Medicago littoralis
green manure was taken up by a wheat crop, whereas 62% of the label
remained in the soil organic fraction. In contrast, an average of 47% of the
labeled N in urea, ammonium sulfate, and potassium nitrate fertilizers was
taken up by wheat,and only 29% remained in the soil organic fraction.Similar
results were obtained for comparisons of N dynamics following application of
green manure and synthetic N fertilizer in wheat fields in Alberta and
Saskatchewan (Janzen et al ., 1990), and maize fields in Pennsylvania (Harris et
al ., 1994).
Release of nutrients from crop residue is not always a slow process,
however. Luna-Orea, Wagger & Gumpertz (1996) measured nutrient release
from two legume cover crops ( Desmodium adscendens and Pueraria phaseoloides ) in