3.2 K cycles in soil-plant systems
K is the seventh-most abundant element in the Earth’s crust and the
second-most abundant nutrient in leaves.
Askegaard,
Eriksen & Johnston (2004) reported
that K concentrations range from 0.4- 4.3% and vary widely with plant
species, year, and fertilizer input. Similarly, appropriate K
concentrations assist in plant photosynthesis, as well as plant
resistance to drought, disease, and salt stress (Römheld & Kirkby,
2010; Malagoli, Britto, Schulze & Kronzucker, 2008; Zhang et al.,
2018). The aim of K fertilization is to reduce crop growth restriction
due to K deficiency. Previous studies have shown that K fertilizer
application improves yields of rice, rapeseed, and wheat by 18.6%,
18.5%, and 17.1%, respectively, on average (Ren et al., 2013; Timsina,
Kumar Singh & Majumdar, 2013).
K in soil is influenced by the tradeoffs between K input and output in
soil-plant systems. Many studies have used K balance as a reflection of
soil K (Singh, Singh & Damodar Reddy, 2002; Yu, Jiang, Zhou & Ma,
2008). During the past 30 years, soil K balance has been highly negative
due to shortages of K input and large crop harvests (Sheldrick, Syers &
Lingard, 2003). Thus, soil available K has increased due to the
transformation of K from a nonexchangeable to an exchangeable state. A
31-year field experiment showed that insufficient K fertilizer
application significantly changed soil quantity/intensity relationship,
and caused depletion of different forms of K (Liao et al., 2017).
Another 3-year experiment presented that soil K depletion firstly
occurred in available K, then in slowly available K (Zhu, Lu, Cong, Ren
& Li, 2019). Many studies indicated that without K fertilization, soil
exchangeable and nonexchangeable K pools were depleted, even with the
presence of K-bearing minerals, since most of the K utilized by crops
comes from the nonexchangeable portion in clay minerals (Andrist-Rangel,
Simonsson, Andersson, Öborn & Hillier, 2006; Zhao et al., 2014), the
ability of 2:1 clay minerals to fix or release K acts as a K reservoir
in soils (Barré, Velde & Abbadie, 2007a;
Barré, Velde, Fontaine, Catel &
Abbadie, 2008). During the period 2006-2019, synthetic K sources,
including K fertilizer, straw and manure, have greatly increased K
input. Soil K balance is positive when K inputs exceed outputs.
Table 2 shows data for soil K balance, soil K content, and clay
mineralogy from long-term K fertilizer experiments. In
treatments
with K and straw, small increases in soil available K were observed at
all sites, which also showed balanced soil K. Lower center of gravity
position (cg) values in treatments with K and straw indicated that
illitization of interstratified illite/smectite clay populations
increased with K addition. Conversely, higher cg values showed that
smectitization interstratified illite/smectite clay populations
decreased with K depletion (Barré, Velde, Fontaine, Catel & Abbadie,
2008). K cycles in soil-plant systems is shown in Figure 2. Crop uptake
and environmental loss are the main fates of fertilizer K. Optimal K
management should aim to balance K application and crop uptake, and
minimize environmental loss while enhancing crop yields.
3.3Environmental
impacts of K changes
3.3.1 Balanced fertilization
Balanced fertilization of low-fertility soils presents a significant
challenge to agriculture. Inputs
of K fertilizer and its proportion
in macroelement fertilizers used in agriculture are shown in Figure 3. K
fertilizer input increased continuously from 1990 to 2005. Since then,
inputs have plateaued. Over the same time period, the proportion of K
fertilizer to total fertilizer input increased continuously from 11.2%
to 31.3%. However, during the period 1990-2005, trend of little or no
use of K has become increasing common. A county-wide survey in the
Yangtze Delta Region by Zhao et al. (2008) found that 38% of samples
had K values below recommended levels. Since 2005, the Chinese
government has conducted soil testing and developed fertilization
technologies, resulting in application of fertilizer based on crop
demands and soil supplying capacity. This has greatly improved soil K
fertility in China (He et al., 2015). Additionally, the application of K
has improved nitrogen and phosphorous use efficiency, increased nutrient
uptake by crops, and further enhanced crop yields (Shukla, Yadav, Singh
& Singh, 2009;
Tan,
Jin, Jiang, Huang & Liu, 2012; Timsina, Kumar Singh & Majumdar, 2013).
A meta-analysis of crop yield responses to different K managements,
using no K fertilizer application as the baseline indicated that K
fertilization, straw return,
and
concurrent
use of both practices increased crop yields (Figure 4). The results
indicated
that K fertilization had positive effects on all crops, rice, wheat, and
rape yields increased by 10.8%, 17.4% and 9.5%, respectively (9.9%
on average). Another meta-analysis evaluated the effects of different K
managements on soil available K content both for one year and several
years. Comprehensive K management
strategies had positive effects on soil available K (42.5% on average;
Figure 5).
The
increase in K fertilization after years of cultivation (49.8%) was
207.4% higher than that for one year (16.2%).
3.3.2
Recycling of straw
Straw has been removed from cropland for livestock or as fuel, a trend
that was difficult to reverse before 2005. However, with recent
increases in the exploitation of coal, petroleum, and natural gas and
the development of industrial feed, farmers have become accustomed to
using new fuels rather than straw, which has made it possible to return
straw to croplands. Straw return increased rice, wheat, and oilseed rape
yields by 6.5%, 7.4%, and 8.4% respectively, and had positive effects
on soil available K (23.8% on average; Figure 4).
Increases in soil available K after
several years of straw return (29.7%) were 403.3% greater than those
for one year (5.9%). Furthermore, K fertilization and straw return
together had even greater improvement on crop yields and soil available
K; rice, wheat, and rape yields increased by 15.5%, 26.9%, and 13.1%,
respectively (16.3% on average for all crops), while soil available K
increased by 66.4%. Increases in soil available K after several years
of cultivation under K fertilization and straw return (82.0%) were
720.0% greater than those for one year (10.0%).
Straw return is the best strategy for improving soil structure, as it
increases soil organic matter fractions and aggregate fractions,
decreases soil bulk density, improves soil C, N, K stocks and nutrients
availability (Singh, Jalota
&
Singh, 2007; Guo & Wang, 2013; Qiu et al., 2014; Wang et al., 2015; Yu
et al., 2016; Zhao et al., 2018). Furthermore, High rates of straw
return changed microbial community structure and promoted soil microbial
activity (Zhao et al., 2016), as well as microbial communities (Pu,
Zhang, Zhang, Liu & Zhang, 2016; Chen et al., 2017; Chen et al., 2017).
3.3.3
Increase
of atmospheric deposition
The availability of K in ecosystems also depends on K inputs via
atmospheric deposition, which can originate both through natural
processes and human and agricultural activities, such as energy
production, transportation, construction-materials production and
fertilization (Walker, Young, Crittenden & Zhang, 2003; Urban et al.,
2012). Golobocanin, Zujic, Milenkovic
&
Miljevic (2009) demonstrated that human activities contribute a higher
proportion of atmospheric deposition than natural processes. Jordi &
Josep (2015) indicated that current levels of K fertilization and
atmospheric deposition of K (0.066 × 109 t
year-1) are very low compared with global available
soil K (57.7 × 109 t). However, in intensive cropping
systems, 4.4-8.2 kg K ha-1 of wet deposition has been
reported in some areas of China (Li & Jin, 2011).
3.3.4 Decrease of soil K fixation
The 2:1 clay minerals play a key role in the soil K cycle (Arkcoll,
Goulding & Hughes, 1985; Hinsinger, 2002). K+ trapped
in 2:1 clay mineral interlayer sites and defined as nonexchangeable
based on classical agronomical tests is available to plants to some
extent (Mortland, Lawton
&
Uehara, 1956; Tributh, Boguslawski, Lieres, Steffens & Mengel, 1987;
Hinsinger, Jaillard & Dufey, 1992). Soil wetting and drying cycles
significantly affect soil K fixation (Zörb, Senbayram
& Peiter, 2014). In soils with
high cation-exchange capacity, wetting and drying cycles increase soil K
fixation (Shakeri & Abtahi, 2019). Soil K fixation also depends on the
K concentration in the soil solution (Schneider, Tesileanu, Charles
&
Sinaj, 2013). Restricting input of K may deplete soil K fertility,
thereby exhausting interlayer K (Mengel & Kirkby, 2001). As interlayer
K is depleted, soil K fixation capacity increases. In K-deficient soils,
or in soils poor in K-rich minerals, K fixation occurs due to the
transfer of K+ from the soil solution to the specific
sites of the 2:1 interlayer, causing their collapse (Portela, Monteiro,
Fonseca & Abreu, 2019). Simonsson, Hillier & Öborn (2009) showed that
the balance between K output and input greatly influenced soil K
fixation capacity. Soils with long-term K application had lower capacity
to fix K compared to those without K treatment (Tan, Liu, Jiang, Luo
&
Li, 2017). Soil K fixation capacity could be reduced after soil K pools
are replenished, thus increasing the availability of fertilizer K, which
would benefit virtuous cycling of
soil K pools.
3.3.5 Reduction of leaching and
runoff loss
K in soil becomes unusable by plants due to limitation in spatial
availability caused by runoff and leaching loss. This is critical for
maintaining soil fertility to reducing environmental loss.
Preferential
flow plays a role in the initial movement of K to depth after a
urination event (Kayser & Isselstein, 2005). A previous study found
that Losses of K by leaching occurred primarily due to macropore flow of
urine below the main rooting depth (Williams, Gregg & Hedley, 1990).
Spikes in nutrient losses frequently occurred following intense rainfall
rather than dry season, especially when coinciding with fertilization
events (Erickson, Cisar, Snyder & Volin, 2005). Over irrigation can
increase risk of soil leaching and runoff. Ma (2004) indicated that
shallow-wet irrigation had higher water use efficiency and less
nutrition loss compared with full-wet irrigation and dry irrigation.
This irrigation mode would be appropriate for sustainable development.
Additionally, soil tillage systems significantly influenced soil K loss.
Soluble K concentrations in runoff water decreased exponentially from
the first to the fifth simulated rainfall test in no-till treatments,
whereas this decrease was linear and less evident in the conventional
tillage treatment (Bertol, Engel, Mafra, Bertol & Ritter, 2007). Under
the paddy-upland cropping system in the Yangtze River basin in China,
wet and dry periods alternate frequently, and unreasonable irrigation
would cause a loss of nutrients, especially during the wet season. Over
the past 15 years, irrigation, soil tillage management, and correct
fertilizer application timing have improved, reducing the risk of
nutrient loss.
4Future
perspectives and conclusions
4.1
Proper K cycles in agricultural systems
China’s agricultural system urgently requires efficient exploitation of
existing K resources and realization of a virtuous cycle for K. Global K
use efficiency for cereal production is only 19% (Dhillon, Eickhoff,
Mullen & Raun, 2019), mainly due to the complete removal of crop
residue. Approximately 80% of K removed from the soil is retained in
nongrain crop residues (Ren et al., 2013; Singh et al., 2018). Improved
returns of crop residue aids in K cycling in agricultural systems.
However, approaches to and effects of straw return and competition with
crops for nitrogen during straw ripening, seeding and the nutrition
absorption require further study. Reducing the risk of K leaching and
runoff loss will have a positive impact on the virtuous cycle of the
soil K reservoir. However, environmental K losses have received less
attention. Virtuous cycles for K can be realized in agricultural systems
by advocating cycles of straw return, developing straw-returning
technology, and improve the machinery and equipment measures that
support straw return.
4.2
Proper
use of K fertilizer
There is a regional imbalance of K input in the Yangtze River basin, and
a large proportion of K fertilizer used in agriculture is imported.
Imbalanced fertilizer use leads to balance surplus of soil N and P but a
serious depletion of K (Zhen, Zoebisch, Chen & Feng, 2006). K
fertilizer application rates have increased dramatically (by 419%) over
the past 30 years, although much of the soil in the Yangtze River basin
is still potentially deficient in K and the soil K pools are hard to
maintain under intensive cropping regimes (He et al., 2015). Therefore,
under condition of high-yield, high-quality, and high-efficiency crop
production, appropriate K input should be considered for the maintenance
and improvement of soil K fertility. Appropriate K fertilization should
take into account crop needs, soil supply, and environmental losses, as
well as fertilizer varieties and the rate, timing, and location of
applications. Inputs of crop residues K also need to be considered to
estimate the potential reductions mineral K fertilizer application when
using straw return, which will be essential for K management of these
rotation systems.
4.3 Conclusions
Over the past 30 years, soil K pools have replenished, and soil
available K increased -7.1%-103.4% across the Yangtze River basin.
These changes were closely related to the changes of agricultural
production conditions as follows: balanced fertilization, including
partial application of N fertilizer and balanced application of NPK
fertilizer; straw return, which return 80% of the total K taken away by
harvest, thus greatly alleviates soil K deficiency; increased soil K
input due to atmospheric deposition and the increase of drying and
wetting deposition; reduction in soil K fixation; and reduction of the
leaching and runoff loss in soil K pools. Although soil K pools have
been improved in the Yangtze River basin in China over the past 30
years, the soil remains in potentially K-deficient state. Realization of
virtuous K cycles in plant-soil system remains a challenge in China, as
does how to determine the appropriate rate for achieving target yields
and improving plant resistance. It is also important to strengthen
relevant support measures in practice. Further improvement in plant-soil
K cycles will be realized in the future in Yangtze River basin in China.