5013.4 Cropping Systems
Crops and cropping systems significantly influence the soil microbial population
and diversity over time. Different workers have reported varying results on the
effects of cropping systems on soil microbial communities. Certain crops may favor
a particular group of microorganisms while others may not exhibit any specific
effect. The potential of crop rotations to increase microbial biomass and activity is
generally agreed. These effects may be mediated through plant root exudates, leaf
leachates from standing biomass, and residues after crop harvest.
Active C fractions, such as MBC, respond quickly to management practices and
better reflect changes in soil quality and productivity that alter nutrient dynamics.
Systems with a short summer fallow are considered superior in maintaining soil
organic matter quantity and activity probably due to reduced soil disturbance and
greater C inputs (Schomberg and Jones 1999 ). Carter ( 1986 ) observed significantly
higher soil microbial biomass carbon (SMBC) in continuous wheat compared with
wheat–fallow. Similarly, Collins et al. ( 1992 ) observed increased SOC, soil organic
nitrogen (SON), MBC and microbial biomass nitrogen (MBN) in long-term con-
tinuous cropping compared to crop–fallow rotations. MBC and MBN increased
when the length of the fallow period decreased in five crop rotations [continuous
spring wheat, spring wheat–fallow, spring wheat–lentil, spring wheat–spring
wheat–fallow, and spring wheat–pea (Pisum sativum L.)–fallow]. Further, the pres-
ence of legumes, such as lentil and pea, in the crop rotation increased soil N frac-
tions (Sainju et al. 2007 ). Venkateswarlu et al. ( 2007 ) examined the possibility of
on-farm generation of legume biomass (horse gram; Macrotyloma uniflorum Lam.)
Verdc.] by using off-season rainfall in two long-term field experiments involving
sorghum (Sorghum bicolor L.) and sunflower. Annual incorporation of horse gram
biomass (3.03–4.28 t fresh weight ha−^1 year−^1 ) improved the soil properties and
fertility status of the soil, which improved the yields of test crops. Biomass incor-
poration improved mean organic carbon (24 %) and MBC (28 %). Long-term bio-
mass incorporation and fertilizer application resulted in a buildup of soil nutrients
compared with fallow plots, resulting in a stable yield trend in sorghum over 10
years, whereas fertilizer application alone showed a declining trend. At the end of
10 years of incorporation, grain yield had increased by 28 and 18 %, respectively,
in sorghum and sunflower (Helianthus annuus L.) compared with the fallow where
no fertilizers were applied to rainy season crops. The incorporation effect was
greater in plots receiving fertilizer. Hence, growing a post-rainy season legume crop
with incorporation is a simple low-cost practice that even small and marginal farm-
ers can adopt in semi-arid regions of India. Widespread adoption of this practice, at
least in alternate years, can restore the productivity of degraded soils and improve
crop yields.
Application of Microbiology in Dryland Agriculture