Millar, N., J. K. Ndufa, G. Cadisch, and E. M. Baggs (2004), Nitrous oxide emissions following incorporation of improved fallow residues in the humid tropics, Global Biogeochem. Cycles, 18, GB1032, doi:10.1029/2003GB002114.
The rotation of crops with fast-growing tree, shrub, and herbaceous N2-fixing
legume species (improved fallows) is a central agroforestry technology
for soil fertility management in the humid tropics. Maize yields are
increased following improved fallows compared with continuous maize
cropping or traditional natural-fallow systems consisting of broadleaved
weeds and grasses. However, the effect of these improved-fallow systems
on N availability and N2O emissions following residue
application has yet to be determined. Emissions from these systems not
only have a detrimental effect on the environment, but are of additional
concern in that they represent a potentially significant loss of N and a
reduction in N-use efficiency. Emissions of N2O were
measured from improved-fallow agroforestry systems in western Kenya,
being characteristic of agroforestry systems in the humid tropics.
Emissions were increased after incorporation of fallow residues and were
higher after incorporation of improved-fallow legume residues (Sesbania sesban, Crotalaria grahamiana, Macroptilium atropurpureum) than natural-fallow residues (mainly consisting of Digitaria abyssibica, Habiscus cannabinus, Bidens pilosa, Guizotia scabra, Leonotis nepetifolia, Commelina benghalensis). Following incorporation of Sesbania and Macroptilium residues (7.4 t dry matter ha−1; 2.9% N) in a mixed fallow system, 4.1 kg N2O-N ha−1 was emitted over 84 days. The percentages of N applied emitted as N2O
following residue incorporation in these tropical agroforestry systems
were of the same magnitude as in temperate agricultural systems. N2O (loge) emissions were positively correlated with residue N content (r = 0.93; P
< 0.05), and thus the residue composition, particularly its N
content, is an important consideration when proposing management
practices to mitigate N2O emissions from these systems.