Agriculture Reference
In-Depth Information
incorporation increased labile C (measured as potassium permanaganate oxidizable) and
mineralizable N in POM in a manner equivalent to manure application (Mtambanengwe
and Mapfumo, 2008). In another study, total soil C, microbial biomass C (MBC), soil C min-
eralization, and soil-specific respiration were all higher in legume-treated soils, although
the difference in MBC was not statistically significant (Kone et al., 2008). Total microbial
biomass increased with legume fallows relative to continuous maize (Nyamadzawo et
al., 2009; Bossio et al., 2005), and 16S ribosomal RNA (rRNA) gene and phospholipid fatty
acid (PLFA) analysis demonstrated that shifts in the microbial community structure also
occurred (Bossio et al., 2005). Seasonal changes in microbial biomass C and N and crop N
uptake led Sugihara et al. (2010) to conclude that soil microbes serve as an important N
source for crop growth during the grain-forming stage in dry tropical cropland in Tanzania.
The effect of residue incorporation on changes in soil C and N pools is complex and
depends in part on soil and residue properties. In a clayey soil, adding organic residues,
irrespective of quality, increased total soil C and N as well as macroaggregates, whereas
residue inputs had little effect in a loamy, sandy soil (Gentile et al., 2010). Conversely,
another study found that in a sandy soil a medium-quality residue (higher polyphenol
content) led to greater amounts of residue-derived N being found in coarse POM as com-
pared to high-quality residue (Chivenge, Vanlauwe, Gentile, et al., 2011), yet residue quality
had no effect on C pools in a clayey soil in the same study. The reason for these differences
across soil types is unclear. Also, soil quality measured as MBC and available P improved
more rapidly at a higher-fertility site with cover crop use than when cover crops were used
at a lower-fertility location (Kone et al., 2008b).
Even when there is no effect of residue quality on long-term soil C accumulation, it
may affect short-term C and N release dynamics and hence synchrony between N release
and crop demand. Another study found that residue quality did not affect long-term soil
C accumulation, but it did affect short-term C and N release dynamics and interactions
with fertilizer additions (Gentile et al., 2011). The combination of fertilizer and low-quality
residue immobilized more fertilizer N than with a high-quality residue. Under field con-
ditions, this temporary immobilization actually reduced N losses and led to a positive
effect on crop N uptake (Gentile et al., 2011). Nitrogen mineralization rates are known to
vary among legume residues of different qualities, with polyphenol content and N content
being the major properties controlling N release (Baijukya et al., 2006). Improved under-
standing of seasonal dynamics of mineralization, microbial populations, and changes
in labile pools of C and N, associated with incorporation of legume residues of different
qualities and fertilizer use, is needed to enhance synchrony between nutrient availability
and crop demand.
More work is also needed to relate changes in different SOM fractions and microbial
populations noted to key soil physical properties, such as improved aggregate stability
and reduced crusting, that are important for reducing soil erosion. Some work has shown
improvements in water infiltration rates (Nyamadzawo et al., 2007), reduced bulk den-
sity, and improved aggregate stability (Sileshi and Mafongoya, 2006) following improved
legume fallows, but little information is available for other systems.
While most research has focused on C and N dynamics, some studies have shown ben-
efits of legume incorporation on crop growth and P uptake (Pypers et al., 2007; Akinnifesi
et al., 2007; Alvey et al., 2001). This may be due to increased microbial mineralization of
soil organic P (Randhawa et al., 2005), solubilization of inorganic P fixed by iron (Fe) and
aluminum (Al) in the soil (Mweta et al., 2007), stimulation of AM infection (Bagayoko et
al., 2000), or a combination of effects (Alvey et al., 2001). Given the low P status of many
African soils, the effects of long-term legume use on P fertility warrant further study.
Search WWH ::
Custom Search