The Myth of Nitrogen Fertilization for Soil Carbon Sequestration

Abstract

Intensive use of N fertilizer, in modern agricultue is motivated by the economic value of high grain yields and is generally perceived to sequester soil organic C by increasing the input of crop residues. This perception is at odds with a century of soil organic C data reported herein for the Morrow Plots, the world's oldest experimental site under continuous corn (Zea mays L.).After 40 to 50 yr of synthetic fertilization that exceeded grain N removal by 60 to 190%, a net decline occurred in soil C despite increasingly massive residue C incorporation, the decline being more extensive for a corn-soybean (Glycine mwv L. Merr.) or corn-oats (Avena sativa L)-hay rotation than for continuous corn and of greater intensity for the profile (0-46 cm) than the surface soil. TIhese findings implicate fertilizer N in promoting the decomposition of crop residues and soil organic matter and are consistent with data from numerous cropping experiments involving synthetic N fertilization in the USA Corn Belt and elsewhere, although not with the interpretation usually provided. There are imporrant implications for soil C sequestration because the yield-based input of fertilizer N has commonly exceeded grain N removal for corn production on fertile soils since the 1960s. To mitigate the ongoing consequences of soil deterioration, atmospheric CO 2 enrichment, and NO,-pollution of ground and surface waters,,N fertilization should be managed by site-specific assessment of soilN availability. Current fertilizer N managemrent practices, if combined with corn stover removal for bioenergy production, exacerbate soil C loss.

The potassium paradox: Implications for soil fertility, crop production and human health

“…Initial recoveries were three-to fivefold higher for non-exchangeable than exchangeable K, followed by a gradual decline toward a stable level of 240-300 kg ha − 1 , with approximately 3000 kg ha − 1 as the cumulative recovery of non-exchangeable K. The resilient behavior of soil K is further revealed, in that 4 years of crop K removal had no consistent effect on soil concentrations of exchangeable, non-exchangeable or total K, implicating the mineral fraction as an important source of buffering. These findings are to be expected, considering what has long been known about the availability and dynamics of non-exchangeable and mineral K, based on chemical extraction 19,[85][86][87] (see also supplemental references [22] and [27][28][29][30] for the online version of the paper), exhaustive cropping 81,86,88,89 (see supplemental references [8], [15] and [16] for the online version of the paper) and electrodialysis 83,90 .…”

“…To allow expression on a mass basis (kg ha − 1 ), test values were multiplied by the corresponding bulk density, obtained from direct measurements in 2005, or by using pedotransfer functions to estimate 1955 values. A more detailed description is available in Khan et al 27 .…”

Carbon management in dryland agricultural systems. A review

“…In drylands, the use of fertilizers is not always followed by an increase of SOC stocks due to the low crop response to the application of nutrients such as N as a consequence of lack of water. As a result, in dryland agriculture, the effects of N fertilization on SOC usually appear in the long term (Álvaro-Fuentes et al 2012) and still are a controversial issue (Khan et al 2007), especially if the energy cost associated with the N fertilizer production is taken into account. In this context, the use of organic fertilizers (i.e., slurries or manures), which is a common practice in some drylands, has the potential to increase SOC stocks and C physical protection within soil aggregates (Plaza-Bonilla et al 2013a).…”

Contribution of nitrogen from sugarcane harvest residues and urea for crop nutrition

“…However, Khan et al (2007) stated that excessive N fertilization (> 200 kg ha -1 year -1 ) could lead to decreases in soil organic C stocks in the long-term due to higher degradation rates of crop residues. Khan et al (2007) also stressed the importance of maintaining part of crop residues even when they could be collected for power generation from biomass or improving sugarcane crop GHG emissions footprints (Macedo et al, 2008).…”