Farming for Ecosystem Services - The Ecology of Agricultural Landscapes - page 40

on average (Robertson 1997). This contrasts with annual inputs of ~7 kg N ha −1
delivered in precipitation at the KBS LTER site.
KBS LTER research has shown that crop management can substantially reduce
long-term nitrate leaching. Over an 11-year period, beginning 6 years after estab-
lishment, the MCSE annual row-crop systems showed 2- to 3-fold differences
in nitrate losses, ranging from average annual losses of 19 and 24 kg N ha −1 in
the Biologically Based and Reduced Input systems, respectively, and of 42 and
62 kg N ha −1 in the No-till and Conventionally managed systems, respectively
(Fig. 2.3; Syswerda et al. 2012). Even after accounting for yield differences
(Fig. 2.1), leaching differences were substantial: 7.3 kg NO 3 − -N per megagram
yield in the Reduced Input system, compared with 11.1 in the No-till and 17.9 in
the Conventional systems.
What accounts for lower nitrate leaching rates? The better soil structure in
No-till cropping systems allows water to leave more quickly (Strudley et al. 2008),
which reduces equilibration with soil microsites where nitrate is formed. But a
more important factor appears to be the presence of cover crops: Even with tillage,
the Reduced Input and Biologically Based systems leached less nitrogen. Cover
crops helped perennialize the crop year; that is, with the fields occupied by growing
plants for a greater proportion of the year, more nitrate is scavenged from the soil
profile and cycled through plant and microbial transformations (McSwiney et al.
2010). More soil water is also transpired, which reduces the opportunity for nitrate
transport: Drainage in the Reduced Input and Biologically Based systems was only
50-70% of that in the Conventional and No-till systems (Fig. 2.3 inset). The rap-
idly growing systems with true perennial vegetation—the Poplar and Successional
Figure 2.3 . Annual nitrate leaching losses and cumulative drainage (inset) from KBS
LTER cropping and successional systems between 1995 and 2006. Modified from Syswerda
et al. (2012).