The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 232

50 kg N ha −1 year −1 (KL-CG50N); the other three were pure ryegrass given
0, 200 and 400 kg N ha −1 year −1 (respectively KL0N, 200N and 400N).
The second site (Kerbernez) was also a well-drained sandy loam soil
of 0.8 m depth, with 4.5% organic matter content and a pH of 5.8. Two
treatments were established: clover-grass receiving no N fertilization
(KZ-CG0N) and pure ryegrass receiving 250 kg N ha −1 year −1 (KZ250N).
At both sites, grasslands were grazed for 6 years by heifers or young
beef cattle. During that period, growth and N accumulated by grass were
measured as well as nitrate leaching (Vertès et al ., 1997; Laurent et al .,
1999). The N balance established over the 6 years varied from

200
(KL0N) to +1200 kg N ha −1 (KL400N). The grasslands were destroyed
in mid-February 1997 by glyphosate application. The amount and bio-
chemical characteristics of grass were determined just before destruction on
all sites.
After destruction, the soil was kept bare (using herbicides) in all treat-
ments for 2 years (1997-1999). Soil cores were taken by auger sampling
every 3 weeks up to 0.8 m depth (in three layers). Water and mineral N
(NH 4 + and NO 3 − ) contents were determined in all soil samples. These
measurements were used in the LIXIM model (Mary et al ., 1999) in order
to calculate water (evaporation and drainage) and nitrogen fluxes (mineral-
ization and leaching). This model searches for the best fit between mea-
sured and simulated water and nitrate contents in each layer, and calculates
mineralization and leaching rates. Lysimeters (Kerbernez) or porous cups
(Kerlavic) were used to determine the amounts of drained water and nitrate
leached.
An incubation experiment was conducted simultaneously (Laurent
et al ., 1998). Soils from each site and treatment were sampled (0-25 cm
depth) just before grass destruction. Aerial parts and roots were separated
from soils, cut into 2-5 mm fragments and added to fresh sieved soil (sieved
on 5-mm mesh). Grass residues were added in amounts similar to field
conditions. Non-treated soils were also incubated. Incubation was per-
formed at 15

−

C and at constant soil water content (80-90% field capacity)
for 1.5 years. Mineralized carbon was monitored continuously by CO 2
trapping. Mineralized nitrogen was determined at regular intervals on soil
samples.

°

In situN dynamics and nitrate leaching

Just before destruction, grass biomass was similar in both sites, ~10 t DM
ha −1 for pure ryegrass and 6.5 t DM ha −1 for clover-grass, corresponding to
140-190 kg N ha −1 and 110-130 kg N ha −1 accumulated in grass residues,
respectively. The C : N ratio of the residues was higher in the unfertilized
treatment (C : N = 30) than in the fertilized grasslands (C : N = 18-24).