Agriculture Reference
In-Depth Information
Decomposition of crop residues and resultant N release are governed by cli-
matic, edaphic, and resource quality factors (Swift et al. 1979).
Of these factors, resource quality is likely the easiest for farmers to manage,
but its impact is often difficult to assess. Farmers use a variety of organic inputs,
ranging from crop residues to manures, which vary widely in N content. And only
a minority of the N from a winter cover crop may be available for the following
summer crop (e.g., 4-35%; Ranells and Wagger 1997). Knowing how the quality
of applied organic materials (e.g., C:N ratio, mineralizable C and N content, and
lignin content) affects N mineralization and immobilization rates (Aulakh et al.
1991, Wagger et al. 1998) is critical for predicting the effect of organic residues on
N availability for annual crops.
Due to its effect on available N, residue quality has also been shown to affect
denitrification rates and N
2
O emissions (Aulakh et al. 2001, Baggs et al. 2000a,
Millar et al. 2004). In laboratory incubations using KBS soils, Ambus et al. (2001)
found that high-quality (1.88% N) pea residues resulted in greater N
2
O emissions
than low-quality (0.63% N) barley residues, when both were separately incorpo-
rated into soil, either as ground or coarsely cut residues. That study also showed
how residue particle size and placement affected both N
2
O emissions and NO
3
−
leaching potentials.
Managing Hydrologic Flow Paths to Retain or Remove Reactive N
Hydrologic export of reactive N from agricultural systems to ground and surface
waters causes well-documented problems, including the degradation of drinking
water by excessively high concentrations of NO
3
−
, eutrophication of downstream
surface waters including marine coastal zones, and additional emission of N
2
O to
the atmosphere (Galloway et al. 2008; Hamilton 2015, Chapter 11 in this volume).
These problems have motivated research to understand how we can manage land-
scapes to retain or remove reactive N from hydrologic flow paths. Three strate-
gies that specifically apply to agricultural watersheds are discussed in this section
(Robertson et al. 2007, Robertson and Vitousek 2009).
First, riparian and other downslope conservation plantings can be managed to
keep NO
3
−
leached from cropped fields from entering local waterways (Liebman
et al. 2013). Native or planted perennial vegetation in stream riparian (buffer) zones
can immobilize N in growing biomass and soil organic matter (Lowrance 1998).
It is well established that waterway grass (filter) strips can also trap soil particles
that would otherwise erode organic N into surface waters. Such measures offer the
additional benefits of mitigating sediment and phosphorus losses to surface waters.
Second, restoring stream channels and small wetlands in agricultural watersheds
can promote denitrification and other microbial processes that convert NO
3
−
to inert
or less mobile forms of N (Mitsch et al. 2001). Denitrification is the main pro-
cess through which streams can permanently remove N (Mulholland et al. 2009).
The effectiveness of wetlands in reducing N export from agricultural fields is
largely dependent on the magnitude and timing of NO
3
−
inputs and the capacity
of the system to denitrify or accumulate N in plant biomass and organic detritus.
Channelization effectively turns headwater streams and wetlands into pipes that
