Maize legume relay intercrops in Malawi - Microbial Ecology in Sustainable Agroecosystems - page 246

Interestingly, we observed higher tissue S levels in the legume plus half fertilizer treat-
ments as compared to unfertilized maize, suggesting that the legumes enhanced S avail-
ability in the soil since the fertilizer applied with the legumes in D2 did not contain any S
(whereas that applied to the full fertilizer MZ+F at planting did). If microbial activity were
increased in the legume treatments, this could have resulted in greater mineralization of
organic S pools in the soil. Studies have shown that crops acquire the majority of their S
from organic forms, even in the presence of sulfate fertilizer (Boye et al., 2010). Alternatively,
it is possible that sulfate leached deeper in the soil profile was accessed by the legumes and
recycled into the upper soil layers. Interactions between legume use and S dynamics war-
rant further investigation. There is also some evidence that inclusion of legumes in rota-
tions can benefit crop P nutrition through the stimulation of AM infection (Bagayoko et al.,
2000; Alvey et al., 2001) or by increasing soil P availability (Randhawa et al., 2005; Mweta et
al., 2007; Alvey et al., 2001). However, we found no evidence for legume-mediated effects on
maize tissue percentage P here ( Table 9.4 ) or soil-available P (see next section).

9.7.1 Soil fertility
Neither treatment nor landscape position resulted in measurable changes in pH and total
soil percentage N in 2000 or 2004 ( Table 9.5 ) , which is perhaps not surprising given the
high variability across sites and the low replicate numbers for soil analysis (12 farmers).
Interestingly, soil percentage C was significantly lower in the dambo margin than on the
hillside in 2000 ( Table 9.6 ), although there were no percentage C treatment effects on maize
yields in 2000 or 2004. The trend toward higher percentage C on the hillside may be related
to the small number of replicates or the more recent transition of agriculture to the hillside
(Banda et al., 1994), whereas the dambo margin has historically been the prime agricultural
land in highly populated southern Malawi. Furthermore, the hillside landscape was often
highly rocky, and farmers pushed together boulders to collect pockets of soil in which to
crop maize (see Figure 9.4 ) , which may have resulted in higher soil C levels in these pock-
ets than in the surrounding soil.
To address the variability issue, we also looked at the percentage change over time
of each parameter for a given plot, and even then there were no discernible changes over
time in either soil percentage C or percentage N or effects of treatment or landscape.
Furthermore, despite the range of soil C and N contents present across the sites, there was
no correlation between maize yields and either percentage C or percentage N ( Table 9.7 ) .
Snapp et al. (2010) also did not detect changes in soil C in Malawi trials of long-lived
legumes that included pigeon pea, noting the long time frame necessary to detect changes

Table 9.5 Two-Way ANOVA Results for Long-Term Soil Indicators for D1 and D2

D1-2000 D2-2004

Legume treatment Landscape Legume treatment Landscape

Variable F P value F P value F P value F P value

pH 1.15 NS 1.6 NS 1.48 NS 3.15 NS

Extractable P
(ppm)

0.07 NS 20.2 0.000*** 0.2 NS 9.65 0.001**

%C-WS 1.13 NS 3.39 0.042* 0.43 NS 2.064 NS

%N-WS 1.41 NS 1.95 NS 0.356 NS 1.47 NS

Note: The asterisks *, **, *** indicate significance at P < 0.05, 0.01, 0.001, respectively. NS, not significant.