Agriculture Reference
In-Depth Information
the soil with no earthworms, or lettuce left on the surface without earthworms to distinguish between
the importance of GAP and NEP. They concluded that GAP reduced microbial biomass and activity,
soil nitrate, and the total N content of the crop, but NEP increased in biological activity and nutrient
availability. It is likely that the relative importance of GAP and NEP will vary among earthworm
species and in soils with different levels of fertility. Indeed, Blair et al. (1995b) noted that stimulatory
effects of earthworms on soil biological and chemical activity appeared to be greater in soils lower
in C and N content.
Blair et al. (1997) suggested a possible mechanism by which earthworm-microbial interactions
can influence soil N availability. In field plots of maize in which earthworm populations were
reduced by 70%, microbial biomass N was significantly higher than in plots with ambient
earthworm populations. After 6 years of manipulating earthworm populations, microbial biomass
C in plots with increased earthworm population was 10% lower than in control plots and 17%
lower than in plots with decreased earthworm populations (Domnguez et al. 2004). Microcosm
experiments yielded similar results; microcosms with earthworms had less microbial biomass N
compared with control microcosms without earthworms (Bohlen and Edwards 1995). Soil nitrate
levels were higher in plots with increased earthworm populations but only in systems fertilized
with inorganic fertilizer, not in systems fertilized with organic fertilizers. Similarly, results from
a long-term tillage experiment in Georgia in the United States, in which
N-labeled crop residues
were added to plots with or without added earthworm populations, demonstrated that earthworm
activity decreased the standing stocks of microbial biomass and increased turnover of microbial
N (Hendrix et al. 1998). Together, these results indicate that earthworms can increase N availability
by reducing microbial immobilization and enhancing mineralization. However, earthworms do
not always reduce microbial biomass, and their influence can vary with soil type or with differences
in the quantity or quality of available organic matter (Wolters and Joergensen 1992; Saetre 1998).
The effects of earthworms on nitrogen cycling were important topics at the Fifth International
Symposium on Earthworm Ecology (Edwards 1998) and continue to be an active area of study.
Bouch et al. (1997) used
15
N to develop models to quantify the direct role of earthworms in the
N cycle. Steinberg et al. (1997) examined the role of earthworms in affecting the availability of N
in forests. They reported that earthworms increased potential net N mineralization rates in both
urban and rural oak forests. In similar forest sites from the same region, Burtelow et al. (1998)
showed that areas with earthworm populations had greater soil denitrification activity than did areas
without earthworms. Other, more recent, studies of earthworm invasion of forests in the northeastern
United States did not report large effects of earthworms on N mineralization (Groffman et al. 2004).
From the studies discussed above, it is apparent that in the majority of cases of earthworm activity
to accelerated transformations of N, often increasing N availability.
At the ecosystem level, there are several possible fates for N that is made available by earth-
15
worms. Some of this N is available for plant uptake (Devliegher and Verstraete 1997; see
Chapter
2
, this volume). However, there is also a possibility of increased losses of N because of increased
leaching and denitrification. Results from our field research in northern Ohio indicated that earth-
worms increased the leaching of N below 45 cm, primarily by increasing the volume of water
leaching through the profile (Domnguez et al. 2004). In a similar study in southern Ohio, there
were larger volumes of leachate at a depth of 45 cm in plots with earthworms added and much
higher concentrations of dissolved organic nitrogen (DON) in the leachate compared with control
plots with ambient earthworm populations (Subler et al. 1997). The effects of earthworms on the
quantity, quality, and movement of soil DON provides another previously unexplored avenue for
research. Increased macropore formation due to earthworm burrows increased leaching (e.g., Lach-
nicht et al. 1997).
The importance of earthworms in soil respiration and organic matter dynamics continues to be
a central theme in investigating the biogeochemical effects of earthworms. Hendriksen (1997) used
pathway analysis to show that earthworms enhanced microbial respiration beyond simply incorpo-
rating dung and increasing soil C and water content. Although earthworms are known to increase
