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Cui et al. Carbon Footprints 2026, 5, 19 Page 3 of 16

MATERIALS AND METHODS
Study region
A long-term field trial was conducted in Huantai County (117°50’00”-118°10’40”E, 36°51’50”-37°06’00”N),
Shandong Province, on the North China Plain. The region is characterized by a temperate continental
monsoon climate, with a mean annual temperature of 11.8-12.9 °C, a mean annual precipitation of 603.9 mm
(primarily from June to August), and a frost-free period of 198 days per year. Before the trial was established,
the farmland had been intensively operated since the early 1980s, when China initiated its reform and
opening-up policy. Two crops, i.e., winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.),
were planted annually in the region. Since the 1980s, crops in the area have been excessively fertilized (~300
kg N per crop season), frequently irrigated, and tilled, with high grain and straw yields (both > 15 t ha yr ).
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The soil is classified as an aquic inceptisol [according to United States Department of Agriculture (USDA)
Soil Taxonomy, the soil is classified as an Inceptisol, suborder Aquepts, and great group Endoaquepts (aquic,
calcareous, clay loam)]. The main properties of the ploughing soil (0-20 cm) in 2008 were a pH of 7.8 (CaCl 2
method), a soil organic carbon content of 10.2 g kg , 30% sand (0.02-2 mm), 32% silt (0.002-0.02 mm), and
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38% clay (< 0.002 mm), and the bulk density was 1.5 g cm .
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Experimental design
From a long-term field trial initiated in 2008 and established using a randomized complete block design, 4
farming treatments (each with four replicates) were selected for this study [Supplementary Figure 1]. Two
crops - winter wheat and summer maize - were planted for the four farming treatments. Each plot covered
450 m (25 m × 18 m), with 5 m buffer strips between treatments and 2 m intervals between subplots.
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(1) Control (CK) treatment: No nitrogen (N) fertilizer was applied during either the wheat or the maize
season. From 2008 to 2024, the average rates of P O and K O in the wheat season were 134 and 59.5 kg ha ,
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2
2
5
respectively, and 102 and 76.9 kg ha , respectively, in the maize season, both of which were applied once as
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basal fertilizer. Straw from both crops was incorporated into the farmland, and rotary tillage was performed
to a depth of 15 cm. Flood irrigation was adopted (75 mm per event) 2-4 times during the wheat season and
0-1 times during the maize season, depending on precipitation and soil moisture. The CK treatment received
59.6 Mg C of crop straw input per hectare.
(2) Local farmer operation treatment (FRM): The N input was 300 kg N ha season for both the wheat and
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maize seasons, which were equally split into basal and topdressing fertilization. Only wheat straw was
incorporated. P and K fertilization and tillage were similar to those in the CK treatment. A similar flood
irrigation treatment to that in the CK treatment was also applied, except with a water amount of 100 mm per
event.

(3) Optimized farming operation treatment (OPT): A soil testing and formula fertilization program was
employed for crop fertilization. A yield of 7.5 t ha for both crops was targeted to determine the crop N
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requirements. The nitrate-N content in the soil was monitored as the soil N supply at the beginning of each
crop season, and the fertilizer N input was calculated as “crop N requirement-soil N supply” . From 2008 to
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2024, the average N input in the OPT treatment was 156 kg N ha during the wheat season and 186 kg N ha -1
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during the maize season, with approximately 1/3 supplied by decomposed animal manure and 2/3 supplied
by mineral fertilizer. The average P O and K O rates during the wheat season were 119 and 60.0 kg ha ,
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5
2
2
respectively, whereas those during the maize season were 85 and 78.7 kg ha , respectively; all the treatments
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were applied as basal fertilizer. Straw of both wheat and maize was incorporated into the farmland, and deep
tillage (~25 cm) was conducted prior to wheat sowing every autumn. Water-saving irrigation (75 mm per
event) was implemented.