Agriculture Reference
In-Depth Information
inorganic N that is also highly susceptible to environmental losses. For example, in pro-
ductive Midwestern prairie soils, N reserves in the top 20 cm of soil are in the range of
4,000 kg N ha -1 , and the annual internal supply of N through mineralization may be more
than 200 kg N ha -1 (Cassman et al., 2002).
This soil-derived N can provide a significant amount of N to crops, but we have not
yet figured out how to synchronize soil N mineralization and plant N demand because
of the diversity and complexity of processes controlling both. For example, the temporal
drivers of decomposition dynamics may be related to but not directly aligned with the
drivers of annual crop productivity. Similarly, the microscale processes that influence the
spatial distribution of N in heterogeneous soils are difficult to manage and are strongly
influenced by factors other than plant growth. Further, limitations to studying soil micro-
bial community structure, size, and function have hampered our ability to understand
N mineralization-immobilization dynamics, and many studies in agricultural systems
have focused on the fate of fertilizers, but far fewer have focused on those fundamental
plant-soil-microbe interactions that influence synchrony (Fierer et al., 2009). Given these
challenges, improving N synchrony in cropping systems is a difficult task; however, it is
arguably the most important objective of agricultural research, if not all of soil ecology,
to better understand, and ultimately manage, N synchrony in agricultural soils. Recent
advances in understanding the spatial and temporal processes that regulate internal N
cycling and availability, including soil biological processes and their interactions with
physical and chemical soil properties, are opening up new ways to think about and study
synchrony. With this information, we need to construct a better understanding of soil
biological communities and associated ecosystem processes that control N mineraliza-
tion-immobilization dynamics, including soil aggregation, sorption, and other physical
processes. In the following sections, we address N mineralization and immobilization
dynamics with particular attention given to the spatial and temporal processes that regu-
late N availability in agroecosystems.
6.6 Nitrogen mineralization-immobilization
Even when N fertilizer applications are high, a significant proportion of the N taken up
by crops is derived from N mineralization. Nitrogen mineralization is the conversion of
complex organic forms of N to more simple molecules that plants can utilize, including
NO 3 - , NH 4 + , and amino acids (Robertson and Groffman, 2007). As such, the process of
N mineralization is inextricably tied to decomposition and the activities of all the soil
organisms involved in that process. In the past, decomposition was often attributed pri-
marily to bacteria, but we now know that other organisms, including archaea and fungi,
as well as the higher soil organisms such as microarthropods, nematodes, and earth-
worms, are also important (Ayres et al., 2009). Indeed, the interactions between trophic
levels, rather than the activity of a single group or organism per se, have a pivotal role
in regulating decomposition processes and nutrient turnover. For example, grazing by
microarthropods can increase microbial diversity and activity, while earthworm activi-
ties can dramatically change the spatial and temporal distribution of microbial activity,
decomposition, and nutrient mineralization (Cragg and Bardgett, 2001; Hättenschwiler
and Gasser, 2005; Milcu et al., 2008).
The grazing of microbial communities by microarthropods and other taxa releases N
in a process that has been described as a “microbial loop” (Clarholm, 1985). In this process,
microarthropods consume organic matter primarily to access the microorganisms coloniz-
ing it. When protozoans and microarthropods graze on microbes, they ingest more N than
Search WWH ::




Custom Search