Maize legume relay intercrops in Malawi - Microbial Ecology in Sustainable Agroecosystems - page 245

is high, N will be released more quickly, whereas residue with a lower N content would
decompose more slowly (Baijukya et al., 2006). If released too quickly, N will be vulnerable
to leaching losses, whereas early maize growth could be reduced if it is released too slowly.
Phiri et al. (1999) found that S. sesban , a moderate-quality residue, mineralized slowly in
southern Malawi, with the strongest correlation between biomass N added and soil min-
eral N occurring 85 days after incorporation. Release dynamics from residues are also
affected by fertilizer additions (Gentile et al., 2011; Kwabiah et al., 1999).
Increased N provision is obviously an important mechanism by which legumes
improve maize yields. High rates of nodulation and nitrogen fixation are desirable, and
one study found that more than 93% of the nitrogen in pigeon pea plants in farmers' fields
in Malawi came from fixation (Adu-Gyamfi et al., 2007), and that annual rates of fixation
ranged between 37 and 117 kg N ha -1 . This is greater than the total N content of pigeon
pea harvested in D2 here, suggesting that growth and nodulation were relatively poor in
most fields in these 2 years. Similarly, the biomass N from T. vogelii and S. sesban were well
below amounts found in some other studies (Akinnifesi et al., 2010). As discussed previ-
ously, it may be beneficial to apply even small amounts of N and P fertilizer, if available,
at maize planting to help stimulate early legume root growth and nodulation (Hardarson
and Atkins, 2007; Kamanga, Waddington, et al., 2010; Kamanga, Whitbread, et al., 2010). It
would also be useful to measure levels of viable rhizobia in the soil and degree of nodula-
tion to determine if inoculant additions would be beneficial.
In addition to N input through BNF, there is evidence that deep-rooted legumes, such
as those used here, can recycle nitrogen that has leached deeper in the soil profile back
into surface soils on incorporation (Akinnifesi et al., 2010; Snapp et al., 1998). Interestingly,
pigeon pea has also been found to improve water availability for associated maize plants
by accessing deep water and subsequently releasing water back into the soil through its
shallower roots (Sekiya and Yano, 2004). Enhanced soil moisture could also benefit the soil
microbial community and hence nutrient mineralization. Further, Makumba et al. (2009)
found that decomposing pigeon pea roots contributed substantially to mineral N in the
soil profile; T. vogelii and S. sesban roots could possibly have similar effects.
Maize tissue percentage N levels were highest in both 2003 and 2004 in the full-fertilizer
treatment, followed by similar but lower levels in the various legume plus half fertilizer
treatments, and were lowest in unfertilized control ( Table 9.4 ). Maize yields were positively
correlated with both tissue percentage N and percentage S ( Table 9.3 ) . While nitrogen is the
most commonly limiting nutrient in the region (Snapp, 1998), Weil and Mughogho (2000)
found that maize yields in Malawi responded to sulfur applications in scenarios in which
adequate N was available. Further, extensive S deficiency has been observed in farmer
fields in West Africa (Nziguheba et al., 2009) and elsewhere (Scherer, 2009).

Table 9.4 Maize Foliar %N and %S by Treatment and Landscape for 2003 and 2004

2003 2004

Cropping system %N %S %N %S %P

MZ+F 2.473 a 0.188 a 2.038 a 0.210 a 0.311

MZ-F 1.342 b 0.121 b 1.262 b 0.133 b 0.250

SS+1/2F 1.828 c 0.145 bc 1.715 c 0.156 b 0.248

TV+1/2F 1.852 c 0.150 c 1.723 c 0.149 b 0.264

PP+1/2F 1.769 c 0.139 bc 1.711 c 0.153 b 0.277

Note: Lowercase letters indicate statistical differences based on one-way ANOVAs ( P < 0.05,
Tukey's post hoc test). Absence of letters indicates the factor was not significant.