Agriculture Reference
In-Depth Information
are less conducive to N retention (Opdyke et al. 2006) because water moves out
faster and its N has less contact with stream edges and sediments, particularly dur-
ing periods of high flow (Peterson et al. 2001, Royer et al. 2006, Alexander et al.
2009). Restoring stream channels and small wetlands to intercept leached N has the
potential to significantly reduce downstream NO
3
−
loadings.
Additionally, the third strategy involves targeting landscape positions that con-
tribute disproportionately to watershed N fluxes (Robertson et al. 2007; Robertson
and Vitousek 2009; Hamilton 2015, Chapter 11 in this volume). It is increasingly
clear that much nonpoint source pollution from agriculture arises from relatively
small fractions of the landscape (Giburek et al. 2002). Planting forage or other
perennial crops such as cellulosic biofuels in these areas (Robertson et al. 2011)
could reduce landscape-level N outputs. Restoring or expanding wetlands in low-
lying areas could even convert these areas from N sources to N sinks.
Tillage Management to Mitigate N Loss
Tillage affects a number of factors that influence N conservation: physical factors
such as soil bulk density, water-holding capacity, drainage, aeration, and aggregate
stability; chemical factors such as C and N stores and availability; and biological
factors such as microbial activity, rates of decomposition, the presence of earth-
worms and other invertebrates, and plant root distributions. Few other management
practices have such far-reaching effects on cropping system N cycling.
Historically, tillage is responsible for most soil organic matter loss in cultivated
ecosystems (Paul et al. 2015, Chapter 5 in this volume) and thereby the loss of most
soil organic N stores. On conversion from native vegetation or long-term fallow,
most of the N initially harvested in subsequent crops is derived from decomposi-
tion (Robertson 1997). Once soil organic N pools are depleted—typically, a few
decades in temperate regions, more quickly in the tropics (Robertson and Grandy
2006)—legumes and fertilizers are required to replace the soil's lost capacity to
supply N. Because herbicides can now provide weed control as effectively as till-
age, conservation tillage (including no-till) can be practiced to conserve organic C
and N in soil and thereby restore many of the fertility benefits of less disturbed soils
(e.g., Franzluebbers and Arshad 1997, Lal 2004).
Although the many benefits of no-till are well known (e.g., Blevins et al. 1977,
Phillips et al. 1980), benefits related to NO
3
−
and N
2
O conservation are less clear.
Comparisons of no-till vs. conventional tillage systems have shown no significant
difference in NO
3
−
leaching (e.g., Cabrera et al. 1999, Mitsch et al. 1999, Smith
et al. 1990), or have demonstrated that no-till leaches either more (Tyler and Thomas
1977, Chichester 1977) or less (Rasse and Smucker 1999, Ogden et al. 1999) NO
3
−
.
Syswerda et al. (2012) argue that much of this ambiguity is due to experiment dura-
tion. They note that most studies last only 2-3 years and begin shortly after no-till
establishment. Short-term studies may mask long-term effects that emerge only
over periods with both higher and lower rainfall levels (e.g., Cabrera et al. 1999).
And studies conducted too soon after a change in management can misrepresent
the long-term effects that emerge after equilibration (Rasmussen et al. 1998). In
addition to short duration, many studies are performed in small plots, which cannot
