Rc. Dalal et al., SUSTAINING PRODUCTIVITY OF A VERTISOL AT WARRA, QUEENSLAND, WITH FERTILIZERS, NO-TILLAGE, OR LEGUMES .1. ORGANIC-MATTER STATUS, Australian journal of experimental agriculture, 35(7), 1995, pp. 903-913
Management practices involving legume leys, grain legumes, and no-till
age and stubble retention, along with nitrogen (N) fertiliser applicat
ion for wheat cropping, were examined for their effectiveness in incre
asing soil organic matter (0-10 cm depth) from 1986 to 1993 in a field
experiment on a Vertisol at Warra, Queensland. The treatments were (i
) grass + legume leys (purple pigeon grass, Setaria incrassata; Rhodes
grass, Chloris gayana; lucerne, Medicago sativa; annual medics, M. sc
utellata and M. truncatula) of 4 years duration followed by continuous
wheat; (ii) 2-year rotation of annual medics and wheat (Triticum aest
ivum cv. Hartog); (iii) 2-year rotation of lucerne and wheat; (iv) 2-y
ear rotation of chickpea (Cicer arietinum cv. Barwon) and wheat; (v) n
o-tillage (NT) wheat; and (vi) conventional tillage (CT) wheat. Fertil
iser N as urea was applied to both NT wheat and CT wheat at 0, 25, and
75 kg N/ha.year. The CT wheat also received N at 12.5 and 25kg N/ha.y
ear. After 4 years, soil organic carbon (C) concentration under grass
+ legume leys increased by 20% (650 kg C/ha.year) relative to that und
er continuous CT wheat. Soil total N increased by 11, 18, and 22% afte
r 2, 3, and 4 years, respectively, under grass + legume leys relative
to continuous CT wheat. These increases in soil organic matter were mo
stly confined to the 0-2.5 cm layer. After the start of wheat cropping
, organic C and total N levels declined steadily but were still higher
than under CT wheat and higher than initial values in December 1985.
Although 2-year rotations of lucerne-wheat and medic-wheat had a small
effect on soil organic C, soil total N concentrations were higher tha
n in the chickpea-wheat rotation and continuous CT wheat from November
1990 to November 1992. Soil under chickpea-wheat rotation had organic
C and total N concentrations similar to continuous CT wheat, although
from the former, about 70 kg/ha.year of extra N was removed in the gr
ain from 1989 to 1993. No-tillage practice had a small effect on soil
organic C, although total N concentration was higher than under CT whe
at in November 1993. These effects were mainly confined to the surface
0-2.5 cm depth. The C to N ratio was only affected in soil under gras
s + legume leys, and no-tillage treatments. These data show that resto
ration of soil organic matter in Vertisol requires grass + legume leys
, primarily due to increased root biomass, although soil total N can b
e enhanced by including legume leys for longer duration in cropping sy
stems in the semi-arid and subtropical environment.